ci: put CI keychain first in codesign search list

codesign resolves signing identities through the user keychain search
list (first match wins) and ignores --keychain for the lookup. This
runner hosts another project whose keychain holds the same Apple
Distribution identity, so when that keychain is locked, codesign fails
with errSecInternalComponent no matter how correctly our own keychain
is set up. Prepend the fresh CI keychain to the search list for the
build and always delete it afterward, which restores the original list.

Also drop the runner-diagnostics step, the GitHub-hosted Ruby PATH
export, and the duplicate CODE_SIGN_IDENTITY param.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitea/workflows/testflight.yml

@@ -9,15 +9,10 @@ on:
 jobs:
   testflight:
     runs-on: macos-arm64
-    defaults:
-      run:
-        shell: bash
 
     steps:
       - name: Checkout
         uses: actions/checkout@v4
-        with:
-          fetch-depth: 0
 
       - name: Setup Ruby
         uses: ruby/setup-ruby@v1
@@ -27,19 +22,7 @@ jobs:
           working-directory: ios
 
       - name: Install XcodeGen
-        run: |
-          set -euo pipefail
-          if ! command -v xcodegen >/dev/null 2>&1; then
-            brew install xcodegen
-          fi
-
-      - name: Runner diagnostics
-        run: |
-          set -euo pipefail
-          whoami
-          printf 'HOME=%s\n' "$HOME"
-          security default-keychain -d user || true
-          security list-keychains -d user || true
+        run: command -v xcodegen >/dev/null 2>&1 || brew install xcodegen
 
       - name: Upload to TestFlight
         working-directory: ios
@@ -53,7 +36,4 @@ jobs:
           SYBIL_BUILD_NUMBER: ${{ github.run_number }}
           FASTLANE_SKIP_UPDATE_CHECK: "1"
           FASTLANE_XCODEBUILD_SETTINGS_TIMEOUT: "120"
-        run: |
-          export PATH="/Users/runner/hostedtoolcache/Ruby/3.1.7/arm64/bin:${PATH}"
-          ruby --version
-          bundle exec fastlane ios beta
+        run: bundle exec fastlane ios beta