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.

contrib/guix/libexec/prelude.bash

@@ -58,3 +58,6 @@ 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}}"