keybot: --clean on build commands clears the build caches

[chrisnojima]

What

Adds --clean to build darwin, build mobile and build android, which sets CLEAN_CACHES=1 for the packaging scripts. The scripts then clear the yarn cache, node_modules and the Go build cache instead of reusing them, for a from-scratch build. The client side is keybase/client#29764, which makes the release builds reuse those caches by default.

• build ios --clean already ran xcodebuild clean (CLEAN). It now also sets CLEAN_CACHES.
• Without --clean, every build command sends CLEAN_CACHES=0, which the scripts treat as unset.
• build mobile --clean sets only CLEAN_CACHES, not CLEAN, so it doesn't add an xcodebuild clean.
• Until perf(packaging): drop go build -a, keep JS deps between builds, no Metro in release mobile builds (untested on build machine) client#29764 lands, the scripts ignore CLEAN_CACHES, so this can merge first.

Testing

• New TestBuildCleanSetsCleanCaches: for each of the four build commands, CLEAN_CACHES is 0 without --clean and 1 with it. It fails if the darwin wiring is removed.
• go test ./... passes, and gofmt and golangci-lint are clean.