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>

pkg/runner/testdata/shells/custom/push.yml

@@ -0,0 +1,14 @@
+on: push
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      # prints version and exits, it's not valid (for github) if {0} is not included
+      - shell: pwsh -v '. {0}'
+        run: ''
+  check-container:
+    runs-on: ubuntu-latest
+    container: ghcr.io/justingrote/act-pwsh:latest
+    steps:
+      - shell: pwsh -v '. {0}'
+        run: ''