Run integration tests on host runner in CI

[rhcarvalho]

Migrate the integration-test-elixir workflow job from an Alpine Docker container to run directly on the ubuntu-24.04 host runner using erlef/setup-beam, matching the environment of mix_test, installer_test, and docs.

Key benefits:

• Adds actions/cache for integration_test/deps and integration_test/_build keyed on integration_test/mix.lock. This avoids recompiling ~50 dependencies on every CI run, saving ~50s of setup time on warm caches.
• Eliminates the apk add system package installation step, as build tools are pre-installed on the Ubuntu runner image.
• Removes background socat proxy bridges; GitHub Actions binds database service container ports directly to localhost.

[rhcarvalho]

CI run with

• cold cache: https://github.com/phoenixframework/phoenix/actions/runs/33681726619/job/100419626476?pr=6831
• warm cache: https://github.com/phoenixframework/phoenix/actions/runs/33683257207/job/100424557512?pr=6831

Note how the "Compile dependencies" step went from 41s down to 11s (some deps still recompiled)

[rhcarvalho]

We're also shaving ~10-20s for not having to pull/setup the Alpine container.

[rhcarvalho]

For reference, the next optimization on top of this would be a small patch to use tmpfs for installer/tmp and integration_test/tmp to improve IO performance.

diff

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c9de83067..aef4d6acd 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -96,6 +96,11 @@ jobs:
         with:
           persist-credentials: false

+      - name: Mount installer/tmp on tmpfs
+        run: |
+          mkdir -p installer/tmp
+          sudo mount -t tmpfs -o size=2G,uid=$(id -u),gid=$(id -g) tmpfs installer/tmp
+
       - name: Set up Elixir
         uses: erlef/setup-beam@54075bcc5e249e4758d363f27d099f55d843f124 # v1.24.1
         with:
@@ -289,6 +294,11 @@ jobs:
         with:
           persist-credentials: false

+      - name: Mount installer/tmp on tmpfs
+        run: |
+          mkdir -p installer/tmp
+          sudo mount -t tmpfs -o size=2G,uid=$(id -u),gid=$(id -g) tmpfs installer/tmp
+
       - name: Set up Elixir
         uses: erlef/setup-beam@54075bcc5e249e4758d363f27d099f55d843f124 # v1.24.1```

[SteffenDE]

@rhcarvalho feel free to go ahead and include the tmpfs mount here