TestScripts/env_var_with_go doesn't take into account GOPROXY=DIRECT

[eclipseo]

Fedora Rawhide, Go 1.13, GOPROXY set to DIRECT:

Testing    in: /builddir/build/BUILD/go-internal-1.3.0/_build/src
         PATH: /builddir/build/BUILD/go-internal-1.3.0/_build/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/sbin
       GOPATH: /builddir/build/BUILD/go-internal-1.3.0/_build:/usr/share/gocode
  GO111MODULE: off
      command: go test -buildmode pie -compiler gc -ldflags "-X github.com/rogpeppe/go-internal/version=1.3.0 -extldflags '-Wl,-z,relro -Wl,--as-needed  -Wl,-z,now -specs=/usr/lib/rpm/redhat/redhat-hardened-ld '"
      testing: github.com/rogpeppe/go-internal
github.com/rogpeppe/go-internal/cache
PASS
ok  	github.com/rogpeppe/go-internal/cache	1.567s
github.com/rogpeppe/go-internal/cmd/testscript
--- FAIL: TestScripts (0.05s)
    --- FAIL: TestScripts/env_var_with_go (0.06s)
        testscript.go:308: 
            # Test passing environment variables to scripts with the go command on PATH
            #
            # Below where we check the output of testscript -v, we have to match against
            # the string literal $WORK because testscript rewrites the actual directory
            # to $WORK. Hence we don't want to expand this script's $WORK in such a comparison.
            #
            # This is the counterpart to env_var_no_go.txt (0.000s)
            # Baseline (0.056s)
            > testscript -v noproxy.txt
            [stdout]
            
            WORK=$WORK
            PATH=/builddir/build/BUILD/go-internal-1.3.0/_build/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/sbin
            HOME=/no-home
            TMPDIR=$WORK/tmp
            devnull=/dev/null
            :=:
            exe=
            GOPATH=$WORK/gopath
            CCACHE_DISABLE=1
            GOARCH=amd64
            GOOS=linux
            GOROOT=/usr/lib/golang
            GOCACHE=/builddir/.cache/go-build
            GOPROXY=direct
            goversion=1.13
            
            > env BANANA
            BANANA=
            > env GOPATH
            GOPATH=$WORK/gopath
            > env GOPROXY
            GOPROXY=direct
            PASS
            > stdout ^BANANA=$
            > stdout '^GOPATH=\$WORK[/\\]gopath'$
            > stdout ^GOPROXY=$
            FAIL: testdata/env_var_with_go.txt:16: no match for `^GOPROXY=$` found in stdout
            
FAIL