nvidia: added open build option

[gfelber]

This pull requests adds the build option open to nvidia package (notable nvidia-dkms) which has a similar result to nvidia-open on different distros. (e.g. usable through ./xbps-src pkg nvidia -o open)
As this is currently disabled by default it shouldn't affect anyone, but it should help with adding the ground work so a potential alternative nvidia-open/nvidia-dkms-open package can be added in the future.

Testing the changes

• I tested the changes in this PR: briefly (1-2 months)

Local build testing

• I built this PR locally for my native architecture, (x86_64-glibc)

Note: that nvidia relies on glibc for it's precompiled libraries

[ahesford]

A build option is not the way to achieve this support. The new driver should either be a subpackage or a wholly separate template; work is ongoing in #56685 and #54593.

[gfelber]

wouldn't it be possible todo sth similar to e.g. gdb-multiarch using a symlink and extra functions instead of adding a lot of redundant code?