Serialize workflows using Nix caching

Alleviates the pressure on the GitHub action cache by only allowing a
single nix-caching job at a time.

Fixes TraceMachina#464
Fixes TraceMachina#611

.github/workflows/image.yaml

@@ -12,6 +12,8 @@ permissions: read-all
 
 jobs:
   publish-image:
+    concurrency:
+      group: uses-nix-caching-{{ github.ref }}
     runs-on: large-ubuntu-22.04
     permissions:
       packages: write
@@ -28,9 +30,9 @@ jobs:
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
 
-      # - name: Cache Nix derivations TODO(aaronmondal): revisit this bit when there are less flakes or it is more understood
-        # uses: >- # Custom commit, last pinned at 2023-11-17.
-          # DeterminateSystems/magic-nix-cache-action@a04e6275a6bea232cd04fc6f3cbf20d4cb02a3e1
+      - name: Cache Nix derivations
+        uses: >- # Custom commit, last pinned at 2023-11-17.
+          DeterminateSystems/magic-nix-cache-action@a04e6275a6bea232cd04fc6f3cbf20d4cb02a3e1
 
       - name: Test image
         run: |

.github/workflows/lre.yaml

@@ -40,6 +40,8 @@ jobs:
            //local-remote-execution/examples:hello_lre"
 
   remote:
+    concurrency:
+      group: uses-nix-caching-{{ github.ref }}
     strategy:
       fail-fast: false
       matrix:

.github/workflows/nix.yaml

@@ -44,6 +44,8 @@ jobs:
           bash -c "bazel test ... --verbose_failures"
 
   nix-cargo:
+    concurrency:
+      group: uses-nix-caching-{{ github.ref }}
     strategy:
       fail-fast: false
       matrix: