[build.ps1] Pin Swift 6.0.0 for MSVC 14.40+ compatibility

[stevapple]

C++ STL from MSVC 14.40 and newer versions require Clang 17+ to build. Swift 5.10.1 is no longer capable, but thankfully we have 6.0 now.

We also replaced the nightly toolchain used by the ARM64 build to a newer one.

[compnerd]

This is not really acceptable, we are using 5.10 explicitly because that is the version that is agreed upon as the baseline for bootstrapping.

[stevapple]

@compnerd The Windows on ARM build is already using (a snapshot of) Swift 6.0. An alternative is to fix MSVC version in CI and document that anyone who wants to build a Windows toolchain either needs to provide a custom Swift 6.0 toolchain or to use MSVC 14.39-.

[etcwilde]

No, we need to maintain the invariant that an older compiler can compile the current compiler. The patch for handling static_assert(false ...) for the new Windows SDK has already been backported and will be part of a future 5.10 release dot release, so we should use that once it is out.