ci: migrate ubuntu-latest -> ${{ fromJSON(vars.CI_LABEL_LINUX_X64) }}

Routes Linux CI jobs via the org-level CI_LABEL_LINUX_X64 variable so the
runner pool can be swapped in one place (e.g. emergency fall-back to
ubuntu-latest if the self-hosted runner is down).

Default value of the variable: ["self-hosted","Linux","X64"] -> evomap-shoot.

Skipped files where ubuntu-latest is load-bearing (glibc pin, cross-platform
matrix, deliberately paid runner) -- those are addressed separately.

Author: forrestlinfeng

.github/workflows/check-links.yml

@@ -14,7 +14,7 @@ permissions:
 
 jobs:
   check:
-    runs-on: ubuntu-latest
+    runs-on: ${{ fromJSON(vars.CI_LABEL_LINUX_X64) }}
     steps:
       - uses: actions/checkout@v6
 

.github/workflows/monitor.yml

@@ -11,7 +11,7 @@ permissions:
 
 jobs:
   monitor:
-    runs-on: ubuntu-latest
+    runs-on: ${{ fromJSON(vars.CI_LABEL_LINUX_X64) }}
     steps:
       - uses: actions/checkout@v6
 

.github/workflows/update-stars.yml

@@ -10,7 +10,7 @@ permissions:
 
 jobs:
   update:
-    runs-on: ubuntu-latest
+    runs-on: ${{ fromJSON(vars.CI_LABEL_LINUX_X64) }}
     steps:
       - uses: actions/checkout@v6