devel/electron{34,36,37,38}: Fix build when rust is not installed in time

VAR!= gets expanded on the first make invocation and because rust is a
build dependency, it might not be installed at that time, so getting
the version of rustc was erratic.

Use gn to get the version instead.

Reported by: mikael (via private email)
Obtained from: https://cgit.freebsd.org/ports/commit/?id=ee86324cade722c6cb8013545e6b8a8c6b05cf82