CI: Use --needed when installing pactoys.

[ghost]

Fix the below error demo by https://tea-ci.org/fracting/MSYS2-packages/117:

warning: pactoys-git-r2.07ca37f-1 is up to date -- reinstalling
resolving dependencies...
looking for conflicting packages...

Packages (1) pactoys-git-r2.07ca37f-1

Total Installed Size:  0.09 MiB
Net Upgrade Size:      0.00 MiB

:: Proceed with installation? [Y/n] 
checking keyring...
checking package integrity...
:: File /var/cache/pacman/pkg/pactoys-git-r2.07ca37f-1-x86_64.pkg.tar.xz is corrupted (invalid or corrupted package (checksum)).
Do you want to delete it? [Y/n] 
error: failed to commit transaction (invalid or corrupted package (checksum))
Errors occurred, no packages were upgraded.

[MSYS2 CI] FAILURE: Updating system failed.