Pin SHELL so tests that rely on this produce deterministic output

[jamielinux]

Fixes #148

$ echo $SHELL
/usr/bin/fish

$ go test ./...
?       github.com/mroth/scmpuff        [no test files]
ok      github.com/mroth/scmpuff/internal/arguments     0.001s
ok      github.com/mroth/scmpuff/internal/cmd   0.090s
?       github.com/mroth/scmpuff/internal/cmd/debug     [no test files]
?       github.com/mroth/scmpuff/internal/cmd/exec      [no test files]
ok      github.com/mroth/scmpuff/internal/cmd/expand    0.001s
ok      github.com/mroth/scmpuff/internal/cmd/inits     (cached)
?       github.com/mroth/scmpuff/internal/cmd/intro     [no test files]
ok      github.com/mroth/scmpuff/internal/cmd/status    0.002s
ok      github.com/mroth/scmpuff/internal/gitstatus     (cached)
ok      github.com/mroth/scmpuff/internal/gitstatus/porcelainv1 0.002s

[mroth]

Thank you! Superseded by #150.