Fix `@netlify/plugin-local-install-core` tests

[ehmicky]

Users can enable the following core plugin in their netlify.toml:

[[plugins]]
package = "@netlify/plugin-local-install-core"

When enabled, npm install or yarn is run inside each local plugin's directory. More background on this feature at https://github.com/netlify/build/pull/1307, https://github.com/netlify/build/issues/1640, https://github.com/netlify/build/issues/1656

Link to source code for this feature.

There are several integration tests for this feature. They rely on calling npm install or yarn. However, doing so is unreliable, as this makes network requests which sometimes fail. This makes CI tests fail randomly (example) (other example). We should fix this.

Note: this seems to also impact Functions automatic installation.