net-im/matterircd: Build with Go 1.26 on quarterly

The 2026Q3 branch still defaults to Go 1.25, while upstream's go.mod
requires Go 1.26, so the build fails with GOTOOLCHAIN=local.

This is a direct commit to the branch: main already defaults to Go 1.26
and builds the port with plain USES=go:modules.

Reported by: Corey Halpin <chalpin@cs.wisc.edu> (maintainer, by mail)
Approved by: osa, vvd (Mentors, implicit)