fix: rework setupShellCommand (#930)

* fix: rework `setupShellCommand`

* move all logic to separate function so we can test that later
* split `step.Shell` and `step.WorkingDirectory` setup into own funcs
* general cleanup of function
* use `ActPath` to not collide with checked out repository
* use `shellquote.Split()` instead of `strings.Fields()` for better command split
* replace single string concat with `fmt`

Signed-off-by: hackercat <me@hackerc.at>

* lint(editorconfig): ignore *_test.go due to mixed style

Signed-off-by: hackercat <me@hackerc.at>

.editorconfig

@@ -12,7 +12,7 @@ indent_size = 4
 indent_style = space
 indent_size = 2
 
-[{Dockerfile,*.md}]
+[{Dockerfile,*.md,*_test.go}]
 indent_style = unset
 indent_size = unset