Always create a new environment block before we spawn a process (#7243)

This commit ensures that we always furnish a new process with the
cleanest, most up-to-date environment variables we can. There is a minor
cost here in that WT will no longer pass environment variables that it
itself inherited to its child processes.

This could be considered a reasonable sacrifice. It will also remove
somebody else's TERM, TERM_PROGRAM and TERM_PROGRAM_VERSION from the
environment, which could be considered a win.

I validated  that GetCurrentProcessToken returns a token we're
_technically able_ to use with this API; it is roughly equivalent to
OpenProcessToken(GetCurrentProcess) in that it returns the current
active _access token_ (which is what CreateEnvironmentBlock wants.)

There's been discussion about doing a 3-way merge between WT's
environment and the new one. This will be complicated and I'd like to
scream test the 0-way merge first ;P

Related to #1125 (but it does not close it or resolve any of the other
issues it calls out.)

Fixes #7239
Fixes #7204 ("App Paths" value creeping into wt's environment)

.github/actions/spell-check/dictionary/apis.txt

@@ -4,6 +4,7 @@ alignof
 bitfield
 bitfields
 CLASSNOTAVAILABLE
+environstrings
 EXPCMDFLAGS
 EXPCMDSTATE
 fullkbd
@@ -46,3 +47,4 @@ STDCPP
 syscall
 tmp
 tx
+userenv