Fix Layout-Headers on CI machines

[NickGerleman]

CI machines pass an environment variable as a custom build property to
MSBuild in order to change intermediate build directory. Rely on this
environment variable if set.

Microsoft Reviewers: Open in CodeFlow

[NickGerleman]

Er, I guess technically not an environment variable but I believe they should be added to the environment when running commands.

[NickGerleman]

Hmm, it looks like we will always skip the LayoutHeaders build step apart from x64 debug in publish, unless an ADO variable is set to enable it. I.e. from what I can tell, without CI changes I don't have a way to evaluate the script output until we're actually publishing.

@JunielKatarn I'm going to set this to AutoMerge and take a look at the publish job output when it passes. This is already broken, so we can't really do more damage at least. I do like the idea about being able to pass a base directory to the PowerShell script as well, just because it makes it a bit less likely we will break if refactoring CI scripts. I will make that change if this one doesn't work.

[ghost]

Hello @NickGerleman!

Because this pull request has the AutoMerge label, I will be glad to assist with helping to merge this pull request once all check-in policies pass.

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