ci: Add NetBSD job

[hebasto]

The master branch @ 1b8d4a6 fails to build on NetBSD.

See: https://github.com/hebasto/libmultiprocess/actions/runs/17175941246/job/48731811650

[DrahtBot]

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[ryanofsky]

Errors all appear to be undefined reference to 'kj::getCaughtExceptionAsKj()' linker errors

[ryanofsky]

I noticed getCaughtExceptionAsKj doesn't seem to be defined if KJ_NO_EXCEPTIONS is defined. Doesn't look like netbsd is defining that though https://cvsweb.netbsd.org/bsdweb.cgi/pkgsrc/devel/capnproto/

[Sjors]

Maybe rebase after #197 for easier review?

[hebasto]

Maybe rebase after #197 for easier review?

Sure! Rebased.

[ryanofsky]

Assuming the netbsd job still fails, I guess there are different directions we may go with this issue:

• If it's possible to fix this with a change in libmultiprocess that would be ideal.
• If that's not possible, then maybe netbsd job could install from capnproto from source and we could report an issue to netbsd.
• If installing capnproto from source doesn't work, this will require a fix in capnproto.

And regardless of these, we may want to add a try_compile test and error with suggested workarounds along the lines of #205

[hebasto]

Assuming the netbsd job still fails, I guess there are different directions we may go with this issue:

• If it's possible to fix this with a change in libmultiprocess that would be ideal.

• If that's not possible, then maybe netbsd job could install from capnproto from source and we could report an issue to netbsd.

• If installing capnproto from source doesn't work, this will require a fix in capnproto.

And regardless of these, we may want to add a try_compile test and error with suggested workarounds along the lines of #205

FWIW, build with depends succeeds: https://github.com/hebasto/bitcoin-core-nightly/actions/runs/17482518540/job/49655572035.