feat: allow overriding runtime path for bundle size check (#652)

Not all modules built with aegir are destined to be consumed in a browser, but as currently implemented the size check will run for every package in a monorepo, which may then fail if one of those packages cannot be built as a browser bundle - if it uses node core modules, for example.

The change here is to allow passing a directory path in which to run the size check, this way monorepos can specify the modules they want tested by using a build matrix:
    
```yaml
on:
  pull_request:
    branches:
      - '*'

name: Bundlesize
jobs:
  check:
    runs-on: ubuntu-latest
    strategy:
      matrix:
        node-version: [12.x]
        project:
          - packages/module-1
          - packages/module-2
          - packages/etc
    steps:
      - uses: actions/checkout@v2
      - name: Use Node.js ${{ matrix.node-version }}
        uses: actions/setup-node@v1
        with:
          node-version: ${{ matrix.node-version }}
      - name: Install dependencies
        run: npm install
      - name: Bundlesize ${{ matrix.project }}
        uses: ipfs/aegir/actions/bundle-size@feat/opt-in-to-size-check
        with:
          project: ${{ matrix.project }}
          github_token: ${{ secrets.GITHUB_TOKEN }}
```

Author: achingbrain

actions/bundle-size/action.yml

@@ -1,9 +1,12 @@
 name: 'Bundle Size'
-description: 'Bundle size action for IPFS repos.'
+description: 'Bundle size action for IPFS repos'
 inputs:
   github_token:
     description: The GITHUB_TOKEN secret
     required: true
+  project:
+    description: A directory to run the bundle check in
+    required: false
 runs:
   using: 'node12'
-  main: 'dist/index.js'
+  main: 'dist/index.js'