Merge #21304: guix: Add guix-clean script + establish gc-root for container profiles

867a5e172a23899a4a70eca4a396c64f1951745e guix: Register garbage collector root for containers (Carl Dong)
8f8b96fb542701b7717683caa3848390b24f77ab guix: Update hint messages to mention guix-clean (Carl Dong)
44f6d4f56b16e1dc5e8a23318b8e7aad0665f178 guix: Record precious directories and add guix-clean (Carl Dong)
84912d4b24382ae022da3a863bd6caa2b8948d94 build: Remove spaces from variable-printing rules (Carl Dong)

Pull request description:

  ```
  guix: Record precious directories and add guix-clean

  Many users have reported problems that stem from having an unclean
  working tree. To that end, I've written a guix-clean script which should
  help reset the working tree while respecting user-specified precious
  directories.

  Precious directories, such as:

  - SOURCES_PATH
  - BASE_CACHE
  - SDK_PATH
  - OUTDIR

  Should be preserved when cleaning the working tree, and are thus
  recorded in ./contrib/guix/var/precious_dirs.

  The ./contrib/guix/guix-clean script is able to parse that file and make
  sure to avoid them when cleaning out the working tree.
  ```

ACKs for top commit:
  laanwj:
    ACK 867a5e172a23899a4a70eca4a396c64f1951745e

Tree-SHA512: c498fad781ff5e6406639df2b91b687fc528273fdf266bcdba8f6eec3b3b37ecce544b6da0252f0b9c6717f9d88e844e4c7b72d1877bdbabfc6871ddd0172af5

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