Fix ccache never being invoked on iOS builds

[janicduplessis]

Summary:

ccache is never invoked on iOS builds, even with USE_CCACHE=1.

set_ccache_compiler_and_linker_build_settings points CC/CXX at ccache-clang.sh and passes the resolved path alongside as a build setting, which the script reads as an environment variable:

exec $CCACHE_BINARY clang "$@"

Xcode does not export build settings into the environment of a compile task, so CCACHE_BINARY is empty during a build and that line word-splits down to exec clang "$@". The build succeeds and looks normal, so it fails silently: you get plain clang plus a shell fork per translation unit, which is slower than leaving ccache off. Logging the variable from inside the launcher during a clean rn-tester build printed an empty value on all 1578 compiler invocations, with 0 cacheable calls.

pod install now generates the launchers into Pods/ with both paths resolved at install time, and points CC/CXX at those. The generated script is the one this removes, with $CCACHE_BINARY replaced by the resolved ccache path and the config path absolute rather than derived from $0:

REACT_NATIVE_CCACHE_CONFIGPATH=/path/to/react-native/scripts/xcode/ccache.conf
# Provide our config file if none is already provided
export CCACHE_CONFIGPATH="${CCACHE_CONFIGPATH:-$REACT_NATIVE_CCACHE_CONFIGPATH}"

exec "/opt/homebrew/bin/ccache" clang "$@"

Pods/ is regenerated by every pod install and is gitignored in the app templates, so the paths cannot go stale and nothing tracked gains a machine-specific value. Dropping the build setting also stops pod install writing an absolute ccache path into a tracked project.pbxproj.

There is deliberately no fallback when the ccache path is missing — silently continuing without ccache is the bug being fixed here, so the build fails with the path in the error and pod install puts it right.

I have only tested command-line xcodebuild, not a build started from Xcode.app.

Changelog:

[IOS] [FIXED] - Actually invoke ccache when it is enabled, and stop writing an absolute ccache path into the Xcode project

Test Plan:

rn-tester, clean Debug build for the iOS simulator, Xcode 26.6, Apple M4:

USE_CCACHE=1 bundle exec pod install
xcodebuild -workspace RNTesterPods.xcworkspace -scheme RNTester \
  -configuration Debug -destination 'generic/platform=iOS Simulator' \
  -derivedDataPath /tmp/rnt build

	wall	ccache -s
before	215s	0 cacheable calls
after, empty cache	210s	1578/1582 cacheable
after, warm cache	19s	1578/1578 hits

Warm run is the same command after rm -rf /tmp/rnt/Build/Intermediates.noindex, so every path stays identical.

ruby utils-test.rb — three tests added for the generated script, covering both compilers and the embedded paths. The suite has 29 pre-existing errors on main, unchanged by this PR.

After pod install, project.pbxproj contains no CCACHE_BINARY; previously two occurrences carrying an absolute path.