Escape $ in pipeline interpolation

Buildkite's interpolator doesn't support shell parameter
expansion (`${VAR#pattern}`).
`$$` passes through as a literal `$` to the shell.

---

Generated with the help of Claude Code, https://claude.ai/code

Co-Authored-By: Claude Code Opus 4.6 <[email protected]>

Author: mokagio

.buildkite/pipeline.yml

@@ -48,5 +48,5 @@ steps:
       if: build.tag =~ /^v/
       command: |
           make build-resources-xcframework REFRESH_L10N=1 REFRESH_JS_BUILD=1
-          bundle exec fastlane publish_to_s3 version:${BUILDKITE_TAG#v}
+          bundle exec fastlane publish_to_s3 version:$${BUILDKITE_TAG#v}
       plugins: *plugins