rustbuild: don't set PYTHON_EXECUTABLE and WITH_POLLY cmake vars since they are no longer supported by llvm

[matthiaskrgr]

This resolves

CMake Warning:
Manually-specified variables were not used by the project:

PYTHON_EXECUTABLE WITH_POLLY 

[rust-highfive]

r? @nikomatsakis

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[Mark-Simulacrum]

@bors r+

Thanks!

[bors]

📌 Commit 0065e33 has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit 0065e33 with merge 46e1e75447da878dc5b678ec835c0144fcfc7e0b...

[rust-log-analyzer]

The job i686-mingw-2 of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

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[bors]

💔 Test failed - checks-actions

[matthiaskrgr]

spurious network failure

T14:56:48.1468797Z �[0m�[0m�[1m�[32m Downloaded�[0m semver v0.9.0 2020-09-08T14:56:48.1574092Z �[0m�[0m�[1m�[32m Downloaded�[0m tracing-subscriber v0.2.11 2020-09-08T14:56:48.1937434Z �[0m�[0m�[1m�[32m Downloaded�[0m smallvec v0.6.13 2020-09-08T14:56:48.2043999Z �[0m�[0m�[1m�[33mwarning�[0m�[1m:�[0m spurious network error (1 tries remaining): failed to get 200 response from `https://crates.io/api/v1/crates/itertools/0.8.2/download`, got 503 2020-09-08T14:56:48.2045897Z �[0m�[0m�[1m�[32m Downloaded�[0m tracing-tree v0.1.5 2020-09-08T14:56:48.2146432Z �[0m�[0m�[1m�[33mwarning�[0m�[1m:�[0m spurious network error (1 tries remaining): failed to get 200 response from `https://crates.io/api/v1/crates/measureme/0.7.1/download`, got 503 2020-09-08T14:56:48.2149029Z �[0m�[0m�[1m�[32m Downloaded�[0m unicode-script v0.5.2 2020-09-08T14:56:48.2343641Z �[0m�[0m�[1m�[31merror�[0m�[1m:�[0m failed to download from `https://crates.io/api/v1/crates/itertools/0.8.2/download` 2020-09-08T14:56:48.2343994Z 2020-09-08T14:56:48.2344157Z Caused by: 2020-09-08T14:56:48.2344363Z failed to get 200 response from `https://crates.io/api/v1/crates/itertools/0.8.2/download`, got 503 2020-09-08T14:56:48.2416120Z command did not execute successfully: "D:\\a\\rust\\rust\\build\\i686-pc-windows-gnu\\stage0\\bin\\cargo.exe" "build" "--target" "i686-pc-windows-gnu" "-Zbinary-dep-depinfo" "-j" "8" "--release" "--locked" "--color" "always" "--features" " llvm" "--manifest-path" "D:\\a\\rust\\rust\\compiler/rustc/Cargo.toml" "--message-format" "json-render-diagnostics" 2020-09-08T14:56:48.2416465Z expected success, got: exit code: 

[Xanewok]

@bors retry