build: explicitly specify the python path

[compnerd]

This ensures that we use python 2.7 when building LLVM. The LLVM
tooling has not yet been ported to python 3, and this breaks the CI
builds which are now finding python 3 via PATH.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[compnerd]

@swift-ci please smoke test

[compnerd]

CC: @shahmishal @jrose-apple @gottesmm

Okay, this should "repair" the CI builds for master-next. It explicitly sets the python to python 2.7 to ensure that we get the right python version so that we can build.

[shahmishal]

@compnerd Thanks!

[compnerd]

Please test with following PRs:
swiftlang/swift-corelibs-libdispatch#505

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 92d10a0be9c912e865bbdd1c60228664e1fb1145

[compnerd]

Please test with following PRs:
swiftlang/swift-corelibs-libdispatch#505

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 2c2cbb0b2c65994cd820e756b1167c0f480ed674

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 2c2cbb0b2c65994cd820e756b1167c0f480ed674

[compnerd]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - d5c4606ebe1dec13356603a578b062211a148656

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - d5c4606ebe1dec13356603a578b062211a148656

[compnerd]

Okay, the Linux side is good now - there are 3 swift test failures, but thats a separate issue.

********************
Failing Tests (3):
    Swift(linux-x86_64) :: Runtime/linux-fatal-backtrace.swift
    Swift(linux-x86_64) :: Fuzzing/fuzzer_test.swift
    Swift(linux-x86_64) :: Fuzzing/fuzzer_test_simpler.swift

[compnerd]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - d5c4606ebe1dec13356603a578b062211a148656

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - d5c4606ebe1dec13356603a578b062211a148656

[compnerd]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 2d8cf9d

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 2d8cf9d

[compnerd]

Okay, this is ready to go in now. The Linux failure is the test failures:

********************
Failing Tests (3):
    Swift(linux-x86_64) :: Runtime/linux-fatal-backtrace.swift
    Swift(linux-x86_64) :: Fuzzing/fuzzer_test.swift
    Swift(linux-x86_64) :: Fuzzing/fuzzer_test_simpler.swift

The Darwin build is failing due to the lldb install rules:

13:07:27 CMake Error at /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-master-next/buildbot_incremental/llvm-macosx-x86_64/lib/cmake/llvm/LLVMDistributionSupport.cmake:24 (message):
13:07:27   Specified distribution component 'debugserver' doesn't have an install
13:07:27   target
13:07:27 Call Stack (most recent call first):
13:07:27   CMakeLists.txt:248 (llvm_distribution_add_targets)
13:07:27 
13:07:27 
13:07:27 CMake Error at /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-master-next/buildbot_incremental/llvm-macosx-x86_64/lib/cmake/llvm/LLVMDistributionSupport.cmake:30 (message):
13:07:27   Specified distribution component 'debugserver' doesn't have an
13:07:27   install-stripped target.  Its installation target creation should be
13:07:27   changed to use add_llvm_install_targets, or you should manually create the
13:07:27   'install-debugserver-stripped' target.
13:07:27 Call Stack (most recent call first):
13:07:27   CMakeLists.txt:248 (llvm_distribution_add_targets)
13:07:27 
13:07:27 
13:07:27 -- Configuring incomplete, errors occurred!

However, this fixes the Python on both, which gets us further along the path of getting master-next fixed.