Merge pull request #66642 from apple/maxd/xctest-python3

[5.8] Call python3 to execute XCTest's build_script.py

A workaround for the fact that fixing this in XCTest's repository (swiftlang/swift-corelibs-xctest#441) is blocked on XCTest's CI being broken for unrelated reasons (and ones which I cannot reproduce).

Risk: low, all other Python scripts in the `swift` and `swift-package-manager` repositories already include this change and use `python3` in both `main` and `release/5.9` branches. This specific change has been on the `main` and `release/5.9` branches for a while now.
Original PRs: #65958 for `main` and #66415 for `release/5.9`
Reviewed by: @compnerd, @al45tair, and @shahmishal 
Resolves: unblocks PRs on the `swift-corelibs-foundation` repository that build XCTest as a part of their CI jobs.
Tests: N/A

Author: MaxDesiatov

utils/build-script-impl

@@ -2418,7 +2418,7 @@ for host in "${ALL_HOSTS[@]}"; do
                       XCTEST_BUILD_ARGS="--release"
                   fi
 
-                  call "${XCTEST_SOURCE_DIR}"/build_script.py \
+                  call python3 "${XCTEST_SOURCE_DIR}"/build_script.py \
                       --swiftc="${SWIFTC_BIN}" \
                       --build-dir="${XCTEST_BUILD_DIR}" \
                       --foundation-build-dir="${FOUNDATION_BUILD_DIR}" \
@@ -2964,7 +2964,7 @@ for host in "${ALL_HOSTS[@]}"; do
                   echo "--- Running tests for ${product} ---"
                   FOUNDATION_BUILD_DIR=$(build_directory ${host} foundation)
                   XCTEST_BUILD_DIR=$(build_directory ${host} xctest)
-                  call "${XCTEST_SOURCE_DIR}"/build_script.py test \
+                  call python3 "${XCTEST_SOURCE_DIR}"/build_script.py test \
                       --swiftc="${SWIFTC_BIN}" \
                       --lit="${LLVM_SOURCE_DIR}/utils/lit/lit.py" \
                       --foundation-build-dir="${FOUNDATION_BUILD_DIR}" \