Fix build-plan causes everything to need recompiling

[edwin0cheng]

This PR fix a bug which cause everything to recompiling after cargo build --build-plan.

Note: This is my first PR in cargo repo, so I please tell me if I missed anything.

CC: @CAD97

[rust-highfive]

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[CAD97]

cc @vlad20012

[CAD97]

The potential issue I see right off the bat is whether this breaks "run build plan" as a way of building. Specifically, if the cache busting behavior is required to make sure an external build system running these steps gets the correct output.

Maybe the solution is to insert the cache busting into the build plan?

[vlad20012]

Great! I just thought about how we can workaround this bug in IntelliJ Rust...

[edwin0cheng]

@CAD97 If I understand correctly :

1. The build-plan should not be executing anything and touch any cache.
2. And I basically touched three parts of code: Two parts are updating for timestamps of files which seem to be harmless. One is preventing to remove a file which is not important in build-plan. (If I understand the comment correctly)

But of course I didn't dig too deep about the whole build-plan flow...

[edwin0cheng]

@vlad20012 This bug is blocking us to implement env! macro in rust-analyzer too :)

[edwin0cheng]

Same as this comment, it looks to be unrelated to my change too. And I tried it for master in my Windows machine, it failed too.

[alexcrichton]

The Cargo team discussed this PR yesterday and we're somewhat hesitant to merge this due to the state of build plans. I've opened #7614 to continue the discussion about that.

[edwin0cheng]

Okay, let this PR closed first and continue the discussion in there.