Bound nix cache size in CI

magic-nix-cache-action writes one GitHub cache entry per nix store path,
which churns through the Actions cache API and leaves over a thousand
entries behind. Swap it for nix-community/cache-nix-action, which keeps
the store as a single bounded entry per job, purging all but the newest.

Keys are scoped by `github.job` because jobs realise different store
closures, and job ids are unique across the workflows that set nix up.

Note this reduces entry count and churn but does not by itself free the
cache quota: Swatinem/rust-cache accounts for 9.52 GiB of the 10 GB
against magic-nix-cache's 1.11 GiB. See #1131 for the numbers.

Author: emmanuelist

.github/actions/setup-nix/action.yml

@@ -9,6 +9,12 @@ runs:
     - name: Install nix
       uses: DeterminateSystems/determinate-nix-action@main
     - name: Use nix cache
-      uses: DeterminateSystems/magic-nix-cache-action@main
+      uses: nix-community/cache-nix-action@v7
       with:
-        use-flakehub: false
+        primary-key: nix-${{ runner.os }}-${{ github.job }}-${{ hashFiles('flake.nix', 'flake.lock') }}
+        restore-prefixes-first-match: nix-${{ runner.os }}-${{ github.job }}-
+        gc-max-store-size: 1G
+        purge: true
+        purge-prefixes: nix-${{ runner.os }}-${{ github.job }}-
+        purge-created: 0
+        purge-primary-key: never

.github/workflows/rust.yml

@@ -30,9 +30,15 @@ jobs:
       - name: "Install nix"
         uses: DeterminateSystems/determinate-nix-action@main
       - name: "Use nix cache"
-        uses: DeterminateSystems/magic-nix-cache-action@main
+        uses: nix-community/cache-nix-action@v7
         with:
-          use-flakehub: false
+          primary-key: nix-${{ runner.os }}-${{ github.job }}-${{ hashFiles('flake.nix', 'flake.lock') }}
+          restore-prefixes-first-match: nix-${{ runner.os }}-${{ github.job }}-
+          gc-max-store-size: 1G
+          purge: true
+          purge-prefixes: nix-${{ runner.os }}-${{ github.job }}-
+          purge-created: 0
+          purge-primary-key: never
       - name: "Add nginxWithStream to PATH"
         run: |
           # This is necessary for payjoin-mailroom integration tests
@@ -52,9 +58,15 @@ jobs:
       - name: "Install nix"
         uses: DeterminateSystems/determinate-nix-action@main
       - name: "Use nix cache"
-        uses: DeterminateSystems/magic-nix-cache-action@main
+        uses: nix-community/cache-nix-action@v7
         with:
-          use-flakehub: false
+          primary-key: nix-${{ runner.os }}-${{ github.job }}-${{ hashFiles('flake.nix', 'flake.lock') }}
+          restore-prefixes-first-match: nix-${{ runner.os }}-${{ github.job }}-
+          gc-max-store-size: 1G
+          purge: true
+          purge-prefixes: nix-${{ runner.os }}-${{ github.job }}-
+          purge-created: 0
+          purge-primary-key: never
       - name: "Run code linting"
         run: nix develop -c ./contrib/lint.sh
       - name: "Run documentation linting"
@@ -80,9 +92,15 @@ jobs:
       - name: "Install nix"
         uses: DeterminateSystems/determinate-nix-action@main
       - name: "Use nix cache"
-        uses: DeterminateSystems/magic-nix-cache-action@main
+        uses: nix-community/cache-nix-action@v7
         with:
-          use-flakehub: false
+          primary-key: nix-${{ runner.os }}-${{ github.job }}-${{ hashFiles('flake.nix', 'flake.lock') }}
+          restore-prefixes-first-match: nix-${{ runner.os }}-${{ github.job }}-
+          gc-max-store-size: 1G
+          purge: true
+          purge-prefixes: nix-${{ runner.os }}-${{ github.job }}-
+          purge-created: 0
+          purge-primary-key: never
       - name: "Add nginxWithStream to PATH"
         run: |
           # This is necessary for payjoin-mailroom integration tests