Add support for specifying npm-version as an option

[felipecrs]

Closes #51

[rwjblue]

Thanks for working on this!

The code looks good, but we need a few test scenarios specifically for npm now.

• Update the test-no-options scenario:

action/.github/workflows/CI.yml

Lines 76 to 78 in ee98e4e

	- run: volta install node@12.16.1 yarn@1.19.1
	- run: tests/check-version.sh 'node' 'v12.16.1'
	- run: tests/check-version.sh 'yarn' '1.19.1'

    - run: volta install node@12.16.1 yarn@1.19.1 npm@6.0.0
    - run: tests/check-version.sh 'node' 'v12.16.1'
    - run: tests/check-version.sh 'yarn' '1.19.1'
    - run: tests/check-version.sh 'npm' '6.0.0'

• Tweak test-specified-node-yarn-overrides-pinned-versions test scenario to mention npm, and check npm version:

action/.github/workflows/CI.yml

Lines 80 to 110 in ee98e4e

	test-specified-node-yarn-overrides-pinned-versions:
	runs-on: "${{ matrix.os }}-latest"
	defaults:
	run:
	working-directory: ./action
	strategy:
	matrix:
	os: [ubuntu, macOS, windows]
	steps:
	- uses: actions/checkout@v2
	with:
	ref: 'branch-for-testing-overriding-pinned-projects-in-ci'
	- uses: actions/checkout@v2
	with:
	path: action
	- run: npm ci
	- run: npm run build
	- uses: ./action
	with:
	node-version: 12.14.0
	yarn-version: 1.22.0
	- run: tests/log-info.sh
	- run: tests/check-version.sh 'node' 'v12.14.0'
	- run: tests/check-version.sh 'yarn' '1.22.0'

• Update this test scenario:

action/.github/workflows/CI.yml

Lines 112 to 133 in ee98e4e

	test-specific-volta-node-yarn:
	runs-on: "${{ matrix.os }}-latest"
	strategy:
	matrix:
	os: [ubuntu, macOS, windows]
	steps:
	- uses: actions/checkout@v1
	- run: npm ci
	- run: npm run build
	- uses: ./
	with:
	volta-version: 0.8.7
	node-version: 12.0.0
	yarn-version: 1.22.0
	- run: tests/log-info.sh
	- run: tests/check-version.sh 'volta' '0.8.7'
	- run: tests/check-version.sh 'node' 'v12.0.0'
	- run: tests/check-version.sh 'yarn' '1.22.0'

[felipecrs]

@rwjblue npm installation seems to fail on Windows... 6.0.0. I'll test with a newer version, just to check.

[felipecrs]

Ok, it worked with npm@7.

But be aware: https://github.com/volta-cli/action/runs/1865368873, it might be an issue in Volta. :)

[rwjblue]

Thank you!!