ci: make inputs.skipNativeInstall do what it says (#69674)
...instead of doing the opposite
before this PR:
- `skipNativeInstall: 'yes'` sets `NEXT_SKIP_NATIVE_POSTINSTALL=''`
- `skipNativeInstall: 'no'` sets `NEXT_SKIP_NATIVE_POSTINSTALL=true`
after this PR:
- `skipNativeInstall: 'yes'` sets `NEXT_SKIP_NATIVE_POSTINSTALL='1'`
- `skipNativeInstall: 'no'` sets `NEXT_SKIP_NATIVE_POSTINSTALL=''`
