[Tests] use --legacy-peer-deps for npm 7

This is needed because we test on eslint < 5, and `@typescript-eslint/parser` has peer dep requirements of eslint >= 5.

See ljharb/eslint-plugin-import@b1aef78

Author: ljharb

.github/workflows/node-4+.yml

@@ -60,7 +60,7 @@ jobs:
       - uses: ljharb/actions/node/run@main
         name: 'npm install && npm run tests-only'
         with:
-          after_install: npm install --no-save "eslint@${{ matrix.eslint }}"
+          after_install: NPM_CONFIG_LEGACY_PEER_DEPS=true npm install --no-save "eslint@${{ matrix.eslint }}"
           node-version: ${{ matrix.node-version }}
           command: 'unit-test'
           after_success: 'npm run coveralls'