CI: Fix check for forgotten // +build js constraints in natives.
CI: Fix check for forgotten // +build js constraints in natives.

This is a fixup for 9a32defc023f83849970b3a1500e223363977b42.

That commit tried to introduce a CI check for catching when .go files
in compiler/natives/src are accidentally missing // +buid js build
constraints.

However, due to the weird Circle CI setup with symlinks, that CI check
wasn't actually going to catch any issues, as proven by test commit
a77449814b9e2a2fd036ad7ef659d1153f15fb24 and that it passed that check.

However, test commit 7aebc06d50e8ee2cae27a39ba68fb4a1a1a92bc0 showed
that relative import path patterns work okay with the Circle CI setup,
so use that instead.

gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs