Merge #21243: ci: Avoid invoking curl on the host

fa330d8 ci: Avoid invoking curl on the host (MarcoFalke)

Pull request description:

  The only requirement for the ci system are the programs `docker.io` and `bash`. However, the mac cross build invokes `curl` on the host. Fix that.

  Before:

  ```
  $ FILE_ENV="./ci/test/00_setup_env_mac.sh" ./ci/test_run_all.sh
  ...
  ./ci/test/05_before_script.sh: line 22: curl: command not found
  ```

  After:

  ```
  ... (command passes)

ACKs for top commit:
  promag:
    ACK fa330d8.

Tree-SHA512: 49120fd671a48a6599dd6c34f6d3502a6e9f84b4476061cab06f55cba374d8188f53b9b41363e90f5fafb0074767b581f30bd2545c0b6934580a7eccfa1ef5c4

Author: fanquake

ci/test/05_before_script.sh

@@ -19,7 +19,7 @@ OSX_SDK_BASENAME="Xcode-${XCODE_VERSION}-${XCODE_BUILD_ID}-extracted-SDK-with-li
 OSX_SDK_PATH="${DEPENDS_DIR}/sdk-sources/${OSX_SDK_BASENAME}"
 
 if [ -n "$XCODE_VERSION" ] && [ ! -f "$OSX_SDK_PATH" ]; then
-  curl --location --fail "${SDK_URL}/${OSX_SDK_BASENAME}" -o "$OSX_SDK_PATH"
+  DOCKER_EXEC curl --location --fail "${SDK_URL}/${OSX_SDK_BASENAME}" -o "$OSX_SDK_PATH"
 fi
 
 if [[ ${USE_MEMORY_SANITIZER} == "true" ]]; then