cmd/go: TestGoEnv failing on Plan 9

[0intro]

Since CL 15457, TestGoEnv is failing on Plan 9.

--- FAIL: TestGoEnv (0.29s)
	go_test.go:260: running testgo [env GOARCH]
	go_test.go:275: standard output:
	go_test.go:276: arm
		
	go_test.go:260: running testgo [env GCCGO]
	go_test.go:275: standard output:
	go_test.go:276: gccgo
		
	go_test.go:260: running testgo [env CGO_CFLAGS]
	go_test.go:275: standard output:
	go_test.go:276: -g -O2
		
	go_test.go:260: running testgo [env CGO_CFLAGS]
	go_test.go:275: standard output:
	go_test.go:276: -foobar
		
	go_test.go:260: running testgo [env CC]
	go_test.go:275: standard output:
	go_test.go:276: 
		
	go_test.go:365: CC not found
	go_test.go:366: pattern gcc not found in standard output
FAIL
FAIL	cmd/go	157.846s

See https://build.golang.org/log/2b5d824ac12e3c635818c518bcc692d1025acb77

[0intro]

The problem is that the CC, GOGCCFLAGS and CXX environment variables are ignored on Plan 9 in mkEnv.

See https://github.com/golang/go/blob/e9ffda4/src/cmd/go/env.go#L65

[gopherbot]

CL https://golang.org/cl/33396 mentions this issue.