move CMAKE_PREFIX_PATH env var handling to environment

This causes the variable to be read up front and stored, rather than be
re-read on each invocation of meson.

This does have two slight behavioral changes. First is the obvious one
that changing the variable between `meson --reconfigure` invocations has
no effect. This is the way PKG_CONFIG_PATH already works. The second
change is that CMAKE_PREFIX_PATH the env var is no longer appended to
the values set in the machine file or on the command line, and is
instead replaced by them. CMAKE_PREFIX_PATH is the only env var in meson
that works this way, every other one is replaced not appended, so while
this is a behavioral change, I also think its a bug fix.

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