Resolve absolute sccache wrapper path on Apple

Apple/NetworkExtension/libburrow/build-rust.sh

@@ -71,6 +71,14 @@ fi
 PROTOC=$(readlink -f $(which protoc))
 CARGO_PATH="$(dirname $PROTOC):$CARGO_PATH"
 
+if [[ -n "${RUSTC_WRAPPER:-}" && "${RUSTC_WRAPPER}" != /* ]]; then
+    WRAPPER_PATH="$(command -v "${RUSTC_WRAPPER}" || true)"
+    if [[ -n "${WRAPPER_PATH}" ]]; then
+        RUSTC_WRAPPER="${WRAPPER_PATH}"
+        CARGO_PATH="$(dirname "${WRAPPER_PATH}"):$CARGO_PATH"
+    fi
+fi
+
 # Run cargo without the various environment variables set by Xcode.
 # Those variables can confuse cargo and the build scripts it runs.
 EXTRA_ENV=()