Fixes <https://bugs.gnu.org/32686>.
Reported by Oleg Pykhalov <go.wigust@gmail.com>.
* guix/build/profiles.scm (ensure-writable-directory): Add #:symlink.
[absolute?]: New procedure.
[unsymlink]: Use it to determine how to resolve readlink's result.
(build-profile): Pass SYMLINK to 'ensure-writable-directory'.
* tests/profiles.scm ("profile-derivation when etc/ is a relative symlink"):
New test.