build: explicitly specify python interpreter

[compnerd]

This ensures that the build system can control the python interpreter
rather than relying on the PATH or the shebang.

[compnerd]

@swift-ci please test

[compnerd]

CC: @akyrtzi @nathawes

[compnerd]

This can be merged separately as it has no dependencies on the build system it will simply ensure that the python interpreter used to invoke build-script.py is used to invoke gyb. This allows the build system to control which python is used to build with. Unfortunately, we cannot replicate the exact logic for controlling the behaviour here as we can with Swift as the build system uses a custom python script.

[akyrtzi]

Hmm, this is not enough, in swift-syntax repo we commit the gyb-generated files into the repo.
We'd need another PR, or extra commit in this PR, that updates the generated files (specifically the Sources/SwiftSyntax/gyb_generated/Misc.swift file which contains the hash output), and merge it at the same time as the swiftlang/swift#32127

[compnerd]

Hmm, lets actually do that in a follow up change that can be committed in sync. master hasn't yet switched to python3 yet.

This is still a bit worrisome since there is no way for us to currently ensure that macOS builds with python3 as there is an issue with some of the builders not finding python3 always (maybe @shahmishal remembers the details here) and we can fallback to python2 if python3 is not found.

[akyrtzi]

We could potentially pick a specific hash algorithm to make it python version independent.

[shahmishal]

We should use xcrun python3 to make sure it picks up the right python on macOS hosts

[compnerd]

@akyrtzi - I like that idea! That certainly would make the transition much simpler. Would you happen to have any specific hash in mind? Would SHA1 be reasonable enough? That is amongst the supported hashes in the Intel SHA extensions enabling a hardware accelerated variant to be implemented on platforms.

[akyrtzi]

have any specific hash in mind? Would SHA1 be reasonable enough?

Any that is available in all platforms would do, SHA1 is fine.