cmd/go: ignore GOFLAGS values without name in go env/bug

This happens with 'go env' and 'go bug'.

If GOFLAGS variable is set to something like '=value',
running `go env` panics with this error message:

    goroutine 1 [running]:
    cmd/go/internal/base.SetFromGOFLAGS(0xd96838)
        cmd/go/internal/base/goflags.go:101 +0x9a7
    main.main()
        cmd/go/main.go:188 +0x755

This happens when the 'name' of the flag is not
specified ('=' or '=value'), with any combination of other flags.
Other commands show this error message:
    go: parsing $GOFLAGS: non-flag

This happens only with 'env' and 'bug' because we have this:
https://go.googlesource.com/go/+/refs/heads/master/src/cmd/go/internal/base/goflags.go#40

New behaviour: ignore the bad flag, since we don't want to report
that with `go env` or `go bug`.

Fixes: #42013
Change-Id: I72602840ca00293d2a92ea28451b75b9799e3d6c
Reviewed-on: https://go-review.googlesource.com/c/go/+/263098


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Matloob <matloob@golang.org>