apkbuild-lint: escape variable for sed replace string

When running apkbuild-lint on the writefreely APKBUILD the sed command failed since the delimiter ~ is in the source variable. Fixing that and also fixing the hypothetical situation of an & in the source variable which is a special character or sed.

I wasn't able to get backslashes working. I think getting them to work would require a lot more work.

Newlines could also be handled easily but I don't expect them in this variable.