test.bash: unset GOROOT to fix Travis-CI

When Travis-CI runs the test.bash, it unnecessarily pollutes the environment
with a GOROOT environment variable, which confuses each of the custom invocations
of different Go toolchains. Rather than setting the GOROOT prior to each
invocation of the Go toolchain, just clear the variable once, and let each
toolchain figure out the root for themselves (which all recent versions of Go
know how to do so correctly).

Change-Id: I45ae9f9f98f5573ff42502b7244c90bf1569f66e
Reviewed-on: https://go-review.googlesource.com/128363
Reviewed-by: Herbie Ong <herbie@google.com>

test.bash

@@ -37,6 +37,10 @@ for GO_VERSION in ${GO_VERSIONS[@]}; do
 	fi
 done
 
+# Travis-CI sets GOROOT, which confuses later invocations of the Go toolchains.
+# Explicitly clear GOROOT, so each toolchain uses their default GOROOT.
+unset GOROOT
+
 # Download dependencies using modules.
 # For pre-module support, dump the dependencies in a vendor directory.
 # TODO: use GOFLAGS="-mod=readonly" when https://golang.org/issue/26850 is fixed.
@@ -54,7 +58,6 @@ export GOPATH=$TEST_DIR/gopath
 # Run tests across every supported version of Go.
 FAIL=0
 for GO_VERSION in ${GO_VERSIONS[@]}; do
-	export GOROOT=$TEST_DIR/go$GO_VERSION
 	GO_BIN=go$GO_VERSION/bin/go
 	function go_build() {
 		echo "$GO_BIN build $@"