VS: Fix crash finding vswhere on 32-bit Windows

Since commit f85913fa08 (VS: Add support for enumerating VS instances
with vswhere, 2022-04-11, v3.24.0-rc1~282^2), if the COM lookup does not
report any VS instances, we fall back to finding vswhere.  However, the
`getenv` call returns nullptr if the `ProgramFiles(x86)` environment
variable is not set.  Update the logic to tolerate not-set environment
variables.  Also check the plain `ProgramFiles` environment variable.

Fixes: #24090