Clarify that package_json_file is relative to GITHUB_WORKSPACE

[chris-martin]

Clarify the description for package_json_file parameter to specify that the path must be relative to the repository root.

If, for example, you give an absolute file path, it doesn't work, because the provided path will always be prefixed by GITHUB_WORKSPACE at

action-setup/src/install-pnpm/run.ts

Line 64 in 41ff726

const content = readFileSync(path.join(GITHUB_WORKSPACE, packageJsonFile), 'utf8');