feat: enable sccache for Darwin (aarch64-darwin) builds

Add sccache support for self-hosted Darwin runners by:
1. Mounting the sccache disk using stickydisk action
2. Configuring proper permissions for the sccache directory
3. Adding extra-sandbox-paths to Nix configuration to allow access

This brings Darwin builds to parity with Linux builds in terms of
sccache caching support.

Author: yvan-sraka

.github/workflows/nix-build.yml

@@ -126,9 +126,28 @@ jobs:
       - name: Checkout Repo
         if: ${{ matrix.attr != '' }}
         uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+      - name: Mount sccache disk
+        if: ${{ matrix.attr != '' && matrix.postgresql_version }}
+        uses: useblacksmith/stickydisk@v1
+        with:
+          key: ${{ github.repository }}-sccache-${{ runner.os }}-${{ runner.arch }}-${{ matrix.cache_key }}
+          path: /nix/var/cache/sccache
       - name: Install nix
         if: ${{ matrix.attr != '' }}
         uses: ./.github/actions/nix-install-self-hosted
+      - name: Configure sccache for Nix builds
+        if: ${{ matrix.attr != '' && matrix.postgresql_version }}
+        run: |
+          # Ensure sccache directory is writable by the build user
+          sudo mkdir -p /nix/var/cache/sccache
+          sudo chown -R $(whoami) /nix/var/cache/sccache
+          sudo chmod -R 755 /nix/var/cache/sccache
+          
+          # Update Nix configuration to allow access to sccache directory in sandbox
+          # Create temporary nix.conf with our additions
+          sudo tee -a /etc/nix/nix.conf > /dev/null <<EOF
+          extra-sandbox-paths = /nix/var/cache/sccache
+          EOF
       - name: nix build
         if: ${{ matrix.attr != '' }}
         shell: bash
@@ -149,9 +168,28 @@ jobs:
       - name: Checkout Repo
         if: ${{ matrix.attr != '' }}
         uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+      - name: Mount sccache disk
+        if: ${{ matrix.attr != '' && matrix.postgresql_version }}
+        uses: useblacksmith/stickydisk@v1
+        with:
+          key: ${{ github.repository }}-sccache-${{ runner.os }}-${{ runner.arch }}-${{ matrix.cache_key }}
+          path: /nix/var/cache/sccache
       - name: Install nix
         if: ${{ matrix.attr != '' }}
         uses: ./.github/actions/nix-install-self-hosted
+      - name: Configure sccache for Nix builds
+        if: ${{ matrix.attr != '' && matrix.postgresql_version }}
+        run: |
+          # Ensure sccache directory is writable by the build user
+          sudo mkdir -p /nix/var/cache/sccache
+          sudo chown -R $(whoami) /nix/var/cache/sccache
+          sudo chmod -R 755 /nix/var/cache/sccache
+          
+          # Update Nix configuration to allow access to sccache directory in sandbox
+          # Create temporary nix.conf with our additions
+          sudo tee -a /etc/nix/nix.conf > /dev/null <<EOF
+          extra-sandbox-paths = /nix/var/cache/sccache
+          EOF
       - name: nix build
         if: ${{ matrix.attr != '' }}
         shell: bash

nix/cargo-pgrx/buildPgrxExtension.nix

@@ -165,7 +165,7 @@ let
     buildPhase = ''
       runHook preBuild
 
-
+      ${lib.optionalString (!stdenv.hostPlatform.isDarwin) ''
       if [[ -d "/nix/var/cache/sccache" && -w "/nix/var/cache/sccache" ]]; then
         echo "sccache: cache directory available, enabling"
         export RUSTC_WRAPPER="${sccache}/bin/sccache"
@@ -174,6 +174,7 @@ let
       else
         echo "sccache: cache directory not available, skipping"
       fi
+      ''}
 
       echo "Executing cargo-pgrx buildPhase"
       ${preBuildAndTest}
@@ -195,10 +196,12 @@ let
         --features "${builtins.concatStringsSep " " buildFeatures}" \
         --out-dir "$out"
 
+      ${lib.optionalString (!stdenv.hostPlatform.isDarwin) ''
       if [[ -n "''${RUSTC_WRAPPER:-}" ]]; then
         echo "sccache stats:"
         ${sccache}/bin/sccache --show-stats
       fi
+      ''}
 
       ${maybeLeaveBuildAndTestSubdir}