fix(openbsd): specify the correct usr_lib_exec path (solves #5992)

[hcartiaux]

Proposed Commit Message

fix(openbsd): specify the correct usr_lib_exec path (solves #5992)

Cf. OpenBSD porting guide (16.), program executables reside in the /usr/local/libexec directory
https://www.openbsd.org/faq/ports/guide.html#PortsChecklist

Fixes GH-5992

Test Steps

Tested with openbsd-cloud-image

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[holmanb]

Please include in this PR a reference to documentation or code.

[hcartiaux]

Please include in this PR a reference to documentation or code.

Added to the commit message, use /usr/local/libexec, official doc:
https://www.openbsd.org/faq/ports/guide.html#PortsChecklist

This change has been tested in an OpenBSD VM.