The snap runtime only supports C.UTF-8 locale, as other locales don't seem to be used now just hardcode it.
Fixes#1643.
Signed-off-by: 林博仁(Buo-ren, Lin) <Buo.Ren.Lin@gmail.com>
Otherwise it will be blocked by Apparmor with the following message:
```
$ shellcheck script
audit: type=1400 audit(TIMESTAMP): apparmor="DENIED" operation="open" profile="snap.shellcheck.shellcheck" name=2F6D656469612F4C696E2D42756F2D52656E2F57696E646F7773205553422F717569636B72756E pid=10175 comm="shellcheck" requested_mask="r" denied_mask="r" fsuid=FSUID ouid=OUID
script: script: openBinaryFile: permission denied (Permission denied)
```
NOTE:
* This plug is not Auto-connect plug, it has to be manually connected by user with `snap connect shellcheck:removable-media :removable-media`
* Currently files under /mnt is not checkable as snapd doesn't provide an interface for it for now.
Refer-to: Interfaces reference - Snaps are universal Linux packages <https://docs.snapcraft.io/reference/interfaces>
Signed-off-by: 林博仁(Buo-Ren Lin) <Buo.Ren.Lin@gmail.com>