Fix getdeps introducting a space after a colon which confuses the parser

Summary:
Fix YAML syntax error in the generated GitHub Actions workflow cachelib/public_tld/.github/workflows/getdeps_linux.yml that prevented the linux CI job from loading (Invalid workflow file ... line 161).

The breakage was on every run: step containing --extra-cmake-defines '{"CMAKE_CXX_COMPILER_LAUNCHER": "sccache"}'. The run: value is an unquoted YAML plain scalar, and the ": " (colon + space) inside the JSON was parsed as a mapping key/value separator, producing mapping values are not allowed here. GitHub reported the failing step's first line (161); the actual offending byte was on line 168 col 196, with the same pattern repeated on all 36 build steps.

Two changes:

  1. Quick fix to the generated workflow (cachelib/public_tld/.github/workflows/getdeps_linux.yml) — rewrite the inline JSON in compact form ({"CMAKE_CXX_COMPILER_LAUNCHER":"sccache"}, no space after :) across all 36 occurrences. This unblocks CI immediately.
  2. Durable fix in the generator (opensource/fbcode_builder/getdeps.py, cmake_arg_for) — pass separators=(",", ":") to json.dumps, so future regenerations of any project's workflow (cachelib, folly, fbthrift, watchman, etc.) emit YAML-safe compact JSON.

Both layers are needed: without (1) CI fails today; without (2) the next time anyone runs update-all-github-actions.sh the bug returns.

Reviewed By: vj7-byte, alikhtarov

Differential Revision: D104263446

fbshipit-source-id: 9ff90b341ba5d1aaf64d10ab77b3f6e3552c2398

Author: r-barnes