🍒 [windows] update to the 6.1.2 toolchain in build.ps1

[charles-zablit]

• Explanation:
  Building the toolchain on ARM64 Windows crashes due to a compiler bug in 6.0.x. The crash was fixed in 6.1.0. This patch fixes the build issue by updating the host compiler to 6.1.2.
  Without this patch, it is currently impossible to build the Swift toolchain on ARM64 Windows.
  This is NOT blocking building the official release, as the CI bots run on x64 and cross compile to ARM64. However, anybody trying to build the toolchain on an ARM64 machine will not be able to do so.

• Scope:
  This change fixes an issue in the Windows build script (build.ps1).

• Issues:

• Original PRs:

  • [windows] update to the 6.1.2 toolchain in build.ps1 #82914
  • 🍒 [windows] update to the 6.1.2 toolchain in build.ps1 #84125

• Risk:
  Low, this updates the host compiler's version from 6.0.1 to 6.1.2.

• Testing:
  Built at desk on an ARM64 machine, and in CI, building and testing on an x64 machine.

• Reviewers:

  • @compnerd
  • @shahmishal

[charles-zablit]

@swift-ci please test