german/hunspell: fix swiss-german dictionary

• The swiss-german dictionary was not properly prepared, as the used sed (/bin/sed) was unable to replace 'ß' by 'ss'.

• Use the shebangfix framework to make it use gsed, which is already depended on.

Approved by: office (fluffy)
PR: 257882

(cherry picked from commit 3b26637cd90d6e9b3f8a392e293cd5e181f51c64)