Configure tar to not set mtime

[khionu]

This PR is fixes #6238. Currently uses a patch on tar, and should not be merged until tar makes a release containing the function set_preserve_mtime.

[rust-highfive]

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[ehuss]

Noted on Discord, the minimal-versions jobs failed because the tar version needs to be bumped to 0.4.17 to pick up the version from git. Presumably once a new tar is published and the version is bumped in Cargo.toml, it should pass.

[alexcrichton]

Ok I've pushed 0.4.18 for tar! I think if that's updated here this should be good to go

[alexcrichton]

@bors: r+

[bors]

📌 Commit 5a59494 has been approved by alexcrichton

[bors]

⌛ Testing commit 5a59494 with merge b5462e609f53efd093f2d297d5680efcc79be5a1...

[bors]

💔 Test failed - status-travis

[Eh2406]

flakey test
@bors: retry

[bors]

⌛ Testing commit 5a59494 with merge 6411d39...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 6411d39 to master...