[beta] Bootstrap beta from the real stable compiler

[alexcrichton]

Routine update from the dev-static urls to the static urls, updating dates as
well. Should be the same artifacts.

[rust-highfive]

r? @nikomatsakis

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[rust-highfive]

Warning

• Pull requests are usually filed against the master branch for this repo, but this one is against beta. Please double check that you specified the right target!

[brson]

@bors r+

[bors]

📌 Commit b526b22 has been approved by brson

[bors]

⌛ Testing commit b526b22 with merge ad48677...

[bors]

💔 Test failed - status-appveyor

[TimNN]

@bors retry

• Tracking issue for spurious network failures on bots #40474

[bors]

⌛ Testing commit b526b22 with merge 185a8b6...

[bors]

💔 Test failed - status-appveyor

[Mark-Simulacrum]

@bors retry

• Spurious failures on AppVeyor due to ar.exe errors #40546 (unfortunately it's back...)

FAILED: lib/libLLVMMSP430AsmPrinter.a 
cmd.exe /C "cd . && "C:\Program Files (x86)\CMake\bin\cmake.exe" -E remove lib\libLLVMMSP430AsmPrinter.a && C:\projects\rust\mingw32\bin\ar.exe qc lib\libLLVMMSP430AsmPrinter.a  lib/Target/MSP430/InstPrinter/CMakeFiles/LLVMMSP430AsmPrinter.dir/MSP430InstPrinter.cpp.obj && C:\projects\rust\mingw32\bin\ranlib.exe lib\libLLVMMSP430AsmPrinter.a && cd ."
C:\projects\rust\mingw32\bin\ar.exe: unable to rename 'lib\libLLVMMSP430AsmPrinter.a'; reason: Permission denied
[1451/1629] Linking CXX static library lib\libLLVMMSP430CodeGen.a
ninja: build stopped: subcommand failed.

[bors]

⌛ Testing commit b526b22 with merge d4e107d...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: brson
Pushing d4e107d to beta...