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>

act/runner/testdata/shells/python/push.yml

@@ -1,9 +1,11 @@
 on: push
+env:
+  MY_SHELL: python
 jobs:
   check:
     runs-on: ubuntu-latest
     steps:
-      - shell: python
+      - shell: ${{ env.MY_SHELL }}
         run: |
           import platform
           print(platform.python_version())
@@ -11,7 +13,16 @@ jobs:
     runs-on: ubuntu-latest
     container: node:12-buster
     steps:
-      - shell: python
+      - shell: ${{ env.MY_SHELL }}
         run: |
           import platform
           print(platform.python_version())
+  check-job-default:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        shell: ${{ env.MY_SHELL }}
+    steps:
+      - run: |
+          import platform
+          print(platform.python_version())