LUCI: Use "os=Linux" dimension to select host bots.

Change-Id: I1fa3ff2a8cca42d06a25f48b309d539dd697486a
diff --git a/tools/luci/config/generated/cr-buildbucket.cfg b/tools/luci/config/generated/cr-buildbucket.cfg
index 3b35718..f8a5337 100644
--- a/tools/luci/config/generated/cr-buildbucket.cfg
+++ b/tools/luci/config/generated/cr-buildbucket.cfg
@@ -283,7 +283,7 @@
     builders {
       name: "host-x86-cms"
       swarming_host: "chromium-swarm.appspot.com"
-      dimensions: "builder:host-x86-cms"
+      dimensions: "os:Linux"
       dimensions: "pool:luci.art.ci"
       recipe {
         name: "art"
@@ -307,7 +307,7 @@
     builders {
       name: "host-x86-debug"
       swarming_host: "chromium-swarm.appspot.com"
-      dimensions: "builder:host-x86-debug"
+      dimensions: "os:Linux"
       dimensions: "pool:luci.art.ci"
       recipe {
         name: "art"
@@ -331,7 +331,7 @@
     builders {
       name: "host-x86-gcstress-debug"
       swarming_host: "chromium-swarm.appspot.com"
-      dimensions: "builder:host-x86-gcstress-debug"
+      dimensions: "os:Linux"
       dimensions: "pool:luci.art.ci"
       recipe {
         name: "art"
@@ -355,7 +355,7 @@
     builders {
       name: "host-x86-ndebug"
       swarming_host: "chromium-swarm.appspot.com"
-      dimensions: "builder:host-x86-ndebug"
+      dimensions: "os:Linux"
       dimensions: "pool:luci.art.ci"
       recipe {
         name: "art"
@@ -379,7 +379,7 @@
     builders {
       name: "host-x86-poison-debug"
       swarming_host: "chromium-swarm.appspot.com"
-      dimensions: "builder:host-x86-poison-debug"
+      dimensions: "os:Linux"
       dimensions: "pool:luci.art.ci"
       recipe {
         name: "art"
@@ -403,7 +403,7 @@
     builders {
       name: "host-x86_64-cdex-fast"
       swarming_host: "chromium-swarm.appspot.com"
-      dimensions: "builder:host-x86_64-cdex-fast"
+      dimensions: "os:Linux"
       dimensions: "pool:luci.art.ci"
       recipe {
         name: "art"
@@ -427,7 +427,7 @@
     builders {
       name: "host-x86_64-cms"
       swarming_host: "chromium-swarm.appspot.com"
-      dimensions: "builder:host-x86_64-cms"
+      dimensions: "os:Linux"
       dimensions: "pool:luci.art.ci"
       recipe {
         name: "art"
@@ -451,7 +451,7 @@
     builders {
       name: "host-x86_64-debug"
       swarming_host: "chromium-swarm.appspot.com"
-      dimensions: "builder:host-x86_64-debug"
+      dimensions: "os:Linux"
       dimensions: "pool:luci.art.ci"
       recipe {
         name: "art"
@@ -475,7 +475,7 @@
     builders {
       name: "host-x86_64-ndebug"
       swarming_host: "chromium-swarm.appspot.com"
-      dimensions: "builder:host-x86_64-ndebug"
+      dimensions: "os:Linux"
       dimensions: "pool:luci.art.ci"
       recipe {
         name: "art"
@@ -499,7 +499,7 @@
     builders {
       name: "host-x86_64-non-gen-cc"
       swarming_host: "chromium-swarm.appspot.com"
-      dimensions: "builder:host-x86_64-non-gen-cc"
+      dimensions: "os:Linux"
       dimensions: "pool:luci.art.ci"
       recipe {
         name: "art"
@@ -523,7 +523,7 @@
     builders {
       name: "host-x86_64-poison-debug"
       swarming_host: "chromium-swarm.appspot.com"
-      dimensions: "builder:host-x86_64-poison-debug"
+      dimensions: "os:Linux"
       dimensions: "pool:luci.art.ci"
       recipe {
         name: "art"
diff --git a/tools/luci/config/main.star b/tools/luci/config/main.star
index 011ac67..144c323 100755
--- a/tools/luci/config/main.star
+++ b/tools/luci/config/main.star
@@ -154,7 +154,12 @@
     refs = ["refs/heads/master-art"],
 )
 
-def ci_builder(name, category, short_name, dimensions={}):
+def ci_builder(name, category, short_name, dimensions=None):
+    dimensions = dimensions or {}  # Initialize if it is None.
+    if category.startswith("host"):
+      dimensions["os"] = ["Linux"]
+    else:
+      dimensions["builder"] = name
     luci.builder(
         name = name,
         bucket = "ci",
@@ -165,9 +170,6 @@
         ),
         dimensions = dimensions | {
             "pool": "luci.art.ci",
-
-            # Some builders require specific hardware, so we make the assignment in bots.cfg
-            "builder": name,
         },
         service_account = "art-ci-builder@chops-service-accounts.iam.gserviceaccount.com",