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

867a5e172a guix: Register garbage collector root for containers (Carl Dong)
8f8b96fb54 guix: Update hint messages to mention guix-clean (Carl Dong)
44f6d4f56b guix: Record precious directories and add guix-clean (Carl Dong)
84912d4b24 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 867a5e172a

Tree-SHA512: c498fad781ff5e6406639df2b91b687fc528273fdf266bcdba8f6eec3b3b37ecce544b6da0252f0b9c6717f9d88e844e4c7b72d1877bdbabfc6871ddd0172af5

contrib/guix/libexec/prelude.bash

@@ -58,3 +58,9 @@ VERSION_BASE="${version_base_prefix}${VERSION}"  # TOP
 DISTSRC_BASE="${DISTSRC_BASE:-${VERSION_BASE}}"
 
 OUTDIR_BASE="${OUTDIR_BASE:-${VERSION_BASE}/output}"
+
+var_base_basename="var"
+VAR_BASE="${VAR_BASE:-${VERSION_BASE}/${var_base_basename}}"
+
+profiles_base_basename="profiles"
+PROFILES_BASE="${PROFILES_BASE:-${VAR_BASE}/${profiles_base_basename}}"