[OpenUPM/Github Packages] Fix npm prepublishOnly script error

Fixed OpenUpm: Build pipeline for version 0.1.1 that shows an error when try run the pipeline job: Publish to OpenUPM

TIP: This release 0.1.3 is just a replacement of 0.1.2 because the OpenUPM pipeline for publishing failed. But In essence, the tags are almost the "same".

The fix consists in rewrite the file .env on [upm] branch to REPOSITORY_ROOT=. only from Github Actions CI, and bypass the prepublishOnly script. This one is required only for manually publishing from a local repo (development environment)

Build pipelines summary from: https://openupm.com/packages/com.benoitfreslon.vibration

Build pipeline failure.

ADDED

• Added new function fixEnvFile() to .github/scripts/functions.sh shell script, in order to rewrite the root .env file
• Added new function checkPkgRoot() to verify and define the $PKG_ROOT environment variable

CHANGED

• Renamed environment variables names to UPPER CASE on .github/workflows/main.yml
• Added .env files, in order to define where is the REPOSITORY_ROOT.

  PS: This is important because, in a embedded package branch ([upm-package-embedded]) with a Unity project, usually the root doesn't contains the UPM package files.

• Updated README.md with npm scripts and node installation documentation