Build: Mac: Keep the signing-related keychain unlocked

This fixes hanging Mac build runs in CI which got stuck as they required
interactive unlocking of the keychain.
This issue only became appareant after the switch to universal builds as
that made the build take longer than the re-lock interval.

Fixes: #2925

.github/autobuild/mac.sh

@@ -52,6 +52,8 @@ prepare_signing() {
     # Set up a keychain for the build:
     security create-keychain -p "${KEYCHAIN_PASSWORD}" build.keychain
     security default-keychain -s build.keychain
+    # Remove default re-lock timeout to avoid codesign hangs:
+    security set-keychain-settings build.keychain
     security unlock-keychain -p "${KEYCHAIN_PASSWORD}" build.keychain
     security import certificate.p12 -k build.keychain -P "${MACOS_CERTIFICATE_PWD}" -T /usr/bin/codesign
     security set-key-partition-list -S apple-tool:,apple:,codesign: -s -k "${KEYCHAIN_PASSWORD}" build.keychain