build/gnu: Only make source checkout files writable.

This is a followup commit to 6129ebddbd.

It was suggested by Ludovic in #guix that it's probably safer to leave files
extracted from a tarball alone.  While at it, guard against possible
exceptions that can happen in the presence of dangling symbolic links, for
example.

* guix/build/gnu-build-system.scm (unpack): Wrap the make-file-writable call
in a false-if-exception handler.  Move the for-each loop under the
file-is-directory? cond branch.

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