make the working directory configurable (#6)

Fixes #4.

Example run (with the line-counting fix (#5) applied): https://github.com/marten-seemann-test/target/actions/runs/1071003167

README.md

@@ -25,3 +25,13 @@ steps:
     with:
       run: go vet ./...
 ```
+
+Optionally, the working directory can be specified, analagously to the [`working-directory` option on `run`](https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#jobsjob_iddefaultsrun):
+```yml
+steps:
+  - name: Code Quality Checks
+    uses: protocol/multiple-go-modules@master
+    with:
+      working-directory: scripts
+      run: go vet ./...
+```

action.yml

@@ -5,11 +5,15 @@ inputs:
   run:
     description: "Command(s) to run"
     required: true
+  working-directory:
+    description: "working directory"
+    required: false
 
 runs:
   using: "composite"
   steps:
     - shell: bash
+      working-directory: ${{ inputs.working-directory }}
       run: |
         status=0
         dirs=$(find . \( -name vendor -o -name '[._].*' -o -name node_modules \) -prune -o -name go.mod -print | sed 's:/go.mod$::')