fix find_vstudio when executing from an already defined MSVC command prompt
fix find_vstudio when executing from an already defined MSVC command prompt

.# Discussion

Older MSVC/VS versions don't define the "VisualStudioVersion" environment variable.
Prior to this fix, when executing within an already prepared environment (eg, the
"Visual Studio Command Prompt") with those older MSVC/VS builds, the search logic would
incorrectly try to search for all installed versions and read their environments. But
since the environment had already been prepped, executing `vsvarsall` for any found
MSVC/VS versions resulted in a superimposed and confused/invalid environment.

For the older MSVC/VS versions without "VisualStudioVersion", this fix does use
heuristics to guess at the correct version (and it's corresponding vsvers value [used as
the results key value]). But, even if the wrong version is guessed, the results from
find_vstudio are clean, without any environmental superposition, and fully usable for
compilation.

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