Provide a Meson devenv, if Meson is new enough
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tests: Move most environment variables to top level
This reduces duplication, and lets us set them from the most logical location: it is the helpers directory that builds our helper executables, so it makes sense for the helpers directory to be responsible for setting SRT_HELPERS_PATH.
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build: Add a Meson devenv if supported
With a new enough Meson release, this lets you run something like
meson devenv -C _build/host bin/steam-runtime-system-info
and have it pick up the correct helpers directory.