Properly deal with build directories containing '~'....Fixes <https://bugs.gnu.org/44626>.
Reported by Vagrant Cascadian <vagrant@debian.org>.
* tests/build-utils.scm ("wrap-script, simple case"): Pass
SCRIPT-CONTENTS to 'display' rather than 'format'.
* gnu/services/base.scm (file-system->shepherd-service-name)
[valid-characters, mount-point]: New variables.
Filter out invalid store file name characters from the mount point of
FILE-SYSTEM.