Backport #10827: Fix environment variable handling when running executables

[mergify]

This fixes a bug where environment variables were duplicated when running executables.

overrideEnv <- fromMaybe [] <$> getEffectiveEnvironment ([("PATH", Just newPath)] ++ envOverrides)
let shellEnv = overrideEnv ++ existingEnv

Since getEffectiveEnvironment already calls getEnvironment internally, if any overrides
are passed then the result is a complete environment. Appending it to
the already existing environment results in duplicated environment variables.

The fix:

• Added getFullEnvironment function to handle the common pattern correctly
• Updated code in Bench, Test/ExeV10, Test/LibV09, and Client/Run to use this function

In the future it would be good to generalise getFullEnvironment
further so it can also handle the addLibraryPath case, which modifies
an environment variable, rather than merely setting or unsetting it.

Fixes #10718.

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This is an automatic backport of pull request #10827 done by [Mergify](https://mergify.com).