Support setting shell via defaults.run (#343)

Note that we don't support the more general use of defaults, just its
run key for setting shell and working directory.

The documentation for defaults.run is here:
  https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#defaultsrun

Fixes #214.

Co-authored-by: Casey Lee <cplee@nektos.com>

pkg/runner/testdata/defaults-run/main.yaml

@@ -0,0 +1,29 @@
+name: defaults-run
+on:
+  - push
+defaults:
+  run:
+    shell: sh
+jobs:
+  without-defaults:
+    runs-on: ubuntu-latest
+    steps:
+    - run: echo $SHELL | grep -v bash || exit 1
+  with-defaults:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        shell: bash
+        working-directory: /tmp
+    steps:
+    - run: |
+        echo $SHELL | grep bash || exit 1
+        [ $(pwd) = /tmp ] || exit 2
+  override-in-step:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        shell: bash
+    steps:
+    - run: echo $SHELL | grep -v bash || exit 1
+      shell: sh