Enabled Nx caching on main

- we should be able to trust Nx enough that we can sustain the build
  cache across commits, which will speed up the workflow because we
  don't need to rebuild our TS projects all the time

.github/workflows/ci.yml

@@ -20,10 +20,6 @@ env:
     ~/.cache/ms-playwright/
   CACHED_BUILD_PATHS: |
     ${{ github.workspace }}/ghost/*/build
-  NX_CACHE_RESTORE_KEYS: |
-    nx-Linux-${{ github.ref }}-${{ github.sha }}
-    nx-Linux-${{ github.ref }}
-    nx-Linux
   NX_REJECT_UNKNOWN_LOCAL_CACHE: 0
 
 concurrency:
@@ -136,7 +132,10 @@ jobs:
         with:
           path: .nxcache
           key: nx-Linux-${{ github.ref }}-${{ env.HEAD_COMMIT }}
-          restore-keys: ${{ env.IS_MAIN == 'false' && env.NX_CACHE_RESTORE_KEYS || 'nx-never-restore'}}
+          restore-keys: |
+            nx-Linux-${{ github.ref }}-${{ env.HEAD_COMMIT }}
+            nx-Linux-${{ github.ref }}
+            nx-Linux
 
       - name: Check dependency cache
         uses: actions/cache@v4