When a project has both npm and pnpm, using pnpm/action-setup will result in an error: `Multiple versions of pnpm specified`.
The previous implementation was only meant to avoid the "ERR_PNPM_BAD_PM_VERSION" error, but it did not take into account the situation of multiple different package managers.
Signed-off-by: Kevin Cui <bh@bugs.cc>
d648c2dd06