Fix #10574 - environment pointer sometimes changes before exec, causing segfault in child process.

[ibuclaw]

No description provided.

[dlang-bot]

Thanks for your pull request, @ibuclaw!

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Testing this PR locally

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dub run digger -- build "master + phobos#10615"

[schveiguy]

This needs to be hand-merged with #10614

[ibuclaw]

@schveiguy likely it was the 24 hours between syncing the repository and creating the merge stable.

[ibuclaw]

#10616.