ci: Use nx cache from master by default (#6732)

Currently, we do not store any cache for the master branch.
When we run a PR, it will try to restore the cache from itself, then from the current branch, then just the newest cache it finds at all.

This means that on a fresh branch, it will restore the cache from the last action that ran - which could be any branch, which means the cache may incorporate changed we don't have, leading to us having to rebuild the cache.

Instead, this makes sure we restore from the master branch first, if possible. This makes it necessary to ensure we actually also _store_ the cache when running on master - we only pass a restore key in that case that will never match anything.

Author: mydea

.github/workflows/build.yml

@@ -40,6 +40,15 @@ env:
 
   BUILD_CACHE_KEY: ${{ github.event.inputs.commit || github.sha }}
 
+  # GH will use the first restore-key it finds that matches
+  # So it will start by looking for one from the same branch, else take the newest one it can find elsewhere
+  # We want to prefer the cache from the current master branch, if we don't find any on the current branch
+  NX_CACHE_RESTORE_KEYS: |
+    nx-Linux-${{ github.ref }}-${{ github.event.inputs.commit || github.sha }}
+    nx-Linux-${{ github.ref }}
+    nx-Linux-refs/heads/master
+    nx-Linux
+
 jobs:
   job_get_metadata:
     name: Get Metadata
@@ -194,18 +203,14 @@ jobs:
         # - on release branches
         # - when PR has `ci-skip-cache` label
         if: |
-          needs.job_get_metadata.outputs.is_master == 'false' &&
           needs.job_get_metadata.outputs.is_release == 'false' &&
           needs.job_get_metadata.outputs.force_skip_cache == 'false'
         with:
           path: node_modules/.cache/nx
-          key: nx-${{ runner.os }}-${{ github.ref }}-${{ env.HEAD_COMMIT }}
-          # GH will use the first restore-key it finds that matches
-          # So it will start by looking for one from the same branch, else take the newest one it can find elsewhere
-          restore-keys: |
-            nx-${{ runner.os }}-${{ github.ref }}-${{ env.HEAD_COMMIT }}
-            nx-${{ runner.os }}-${{ github.ref }}
-            nx-${{ runner.os }}
+          key: nx-Linux-${{ github.ref }}-${{ env.HEAD_COMMIT }}
+          # On master branch, we want to _store_ the cache (so it can be used by other branches), but never _restore_ from it
+          restore-keys:
+            ${{needs.job_get_metadata.outputs.is_master == 'false' && env.NX_CACHE_RESTORE_KEYS || 'nx-never-restore'}}
 
       - name: Build packages
         # Under normal circumstances, using the git SHA as a cache key, there shouldn't ever be a cache hit on the built