Cache Zig package downloads in CI to prevent TLS flakes (#2423)

* Cache Zig package downloads in CI to prevent TLS flakes

The ui-regressions job fails intermittently with TlsInitializationFailed
when xcodebuild's Zig build phase fetches dependencies from
deps.files.ghostty.org. Unlike Swift packages (cached + retried), Zig
package downloads had no caching. Add ~/.cache/zig cache steps to all
four Zig-using jobs (tests, tests-build-and-lag, ui-regressions in
ci.yml, and build-ghosttykit.yml) keyed on the ghostty build.zig.zon
files.

* Use shared Zig cache key prefix across all CI jobs

Whichever job runs first warms the cache for all others,
since all jobs fetch the same Zig packages.

---------

Co-authored-by: Lawrence Chen <lawrencecchen@users.noreply.github.com>

.github/workflows/build-ghosttykit.yml

@@ -60,6 +60,14 @@ jobs:
           export DEVELOPER_DIR="$XCODE_DIR"
           xcodebuild -version
 
+      - name: Cache Zig packages
+        if: steps.check-release.outputs.exists == 'false'
+        uses: actions/cache@5a3ec84eff668545956fd18022155c47e93e2684 # v4
+        with:
+          path: ~/.cache/zig
+          key: zig-packages-${{ hashFiles('ghostty/build.zig.zon', 'ghostty/build.zig.zon.json') }}
+          restore-keys: zig-packages-
+
       - name: Build GhosttyKit.xcframework
         if: steps.check-release.outputs.exists == 'false'
         run: |

.github/workflows/ci.yml

@@ -129,6 +129,13 @@ jobs:
             zig version
           fi
 
+      - name: Cache Zig packages
+        uses: actions/cache@5a3ec84eff668545956fd18022155c47e93e2684 # v4
+        with:
+          path: ~/.cache/zig
+          key: zig-packages-${{ hashFiles('ghostty/build.zig.zon', 'ghostty/build.zig.zon.json') }}
+          restore-keys: zig-packages-
+
       - name: Cache DerivedData
         uses: actions/cache@5a3ec84eff668545956fd18022155c47e93e2684 # v4
         with:
@@ -293,6 +300,13 @@ jobs:
             zig version
           fi
 
+      - name: Cache Zig packages
+        uses: actions/cache@5a3ec84eff668545956fd18022155c47e93e2684 # v4
+        with:
+          path: ~/.cache/zig
+          key: zig-packages-${{ hashFiles('ghostty/build.zig.zon', 'ghostty/build.zig.zon.json') }}
+          restore-keys: zig-packages-
+
       - name: Cache DerivedData
         uses: actions/cache@5a3ec84eff668545956fd18022155c47e93e2684 # v4
         with:
@@ -447,6 +461,13 @@ jobs:
             zig version
           fi
 
+      - name: Cache Zig packages
+        uses: actions/cache@5a3ec84eff668545956fd18022155c47e93e2684 # v4
+        with:
+          path: ~/.cache/zig
+          key: zig-packages-${{ hashFiles('ghostty/build.zig.zon', 'ghostty/build.zig.zon.json') }}
+          restore-keys: zig-packages-
+
       - name: Cache Swift packages
         uses: actions/cache@5a3ec84eff668545956fd18022155c47e93e2684 # v4
         with: