cmd/dist: consistently use $GOROOT/bin/go instead of just "go"

Also remove existing special cases that transform "go" into
gorootBinGo, because they make debugging and code-reviews more
difficult: log messages that don't include the full path can mask bugs
like #31567, and the reader of the code has to trace through the
various call chains to verify that the correct "go" is being used.

Instead, we can make the use of the correct "go" command plainly
obvious in the code by using one consistent name for it.
(Prior to this CL, we had three different names for it:
gorootBinGo, "go", and cmdGo. Now we have only one.

Updates #31567.

Change-Id: Ia9ff27e5e800c79af5a4e9f2803c9ea5ccafbf35
Reviewed-on: https://go-review.googlesource.com/c/go/+/452678
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>

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