Use runners from vars if available (#812)

This was removed in #801 to bring the file in line with downstream.
However, it is better to rely on the vars environment when invoking the
main workflow from dispatch, while offering an override when invoking
the workflow as a sub-workflow.

Author: Steelskin

.github/workflows/build-toolchain.yml

@@ -36,17 +36,6 @@ on:
         default: true
         required: false
 
-      windows_default_runner:
-        description: 'Build runner'
-        default: 'windows-latest'
-        required: false
-        type: string
-
-      windows_compilers_runner:
-        description: 'Build runner for `compilers` job'
-        type: string
-        required: false
-
   workflow_call:
     inputs:
       swift_version:
@@ -84,7 +73,6 @@ on:
 
       windows_default_runner:
         description: 'Build runner'
-        default: 'windows-latest'
         required: false
         type: string
 
@@ -267,8 +255,8 @@ jobs:
             fi
           fi
 
-          echo windows_build_runner=${{ inputs.windows_default_runner }} >> ${GITHUB_OUTPUT}
-          echo compilers_build_runner=${{ inputs.windows_compilers_runner || inputs.windows_default_runner }} >> ${GITHUB_OUTPUT}
+          echo windows_build_runner=${{ inputs.windows_default_runner || vars.WINDOWS_BUILD_RUNNER || 'windows-latest' }} >> ${GITHUB_OUTPUT}
+          echo compilers_build_runner=${{ inputs.windows_compilers_runner || inputs.windows_default_runner || vars.COMPILERS_BUILD_RUNNER || vars.WINDOWS_BUILD_RUNNER || 'windows-latest' }} >> ${GITHUB_OUTPUT}
 
           echo ANDROID_API_LEVEL=${{ inputs.android_api_level }} >> ${GITHUB_OUTPUT}