store-copy: Canonicalize the mtime and permissions of the store copy.

store-copy: Canonicalize the mtime and permissions of the store copy.

Fixes a bug whereby directories in the output of 'guix pack -f tarball'
would not be read-only.

* guix/build/store-copy.scm (reset-permissions): New procedure.
(populate-store): Pass #:keep-mtime? #t to 'copy-recursively'.  Call
'reset-permissions'.
* tests/pack.scm ("self-contained-tarball"): In CHECK, define
'canonical?' and use it to check that every file has an mtime of 1 and
is read-only.
* tests/guix-pack.sh: Invoke "chmod -Rf +w" before "rm -rf" in trap.