build: use the correct install location

Use the correct installation location path computation
`toolchain_install_path` rather than trying to compute it ourselves.
This ensures that the files are installed into the right location.

Author: compnerd

utils/swift_build_support/swift_build_support/products/pythonkit.py

@@ -53,7 +53,7 @@ def build(self, host_target):
                 '-G', 'Ninja',
                 '-D', 'BUILD_SHARED_LIBS=YES',
                 '-D', 'CMAKE_INSTALL_PREFIX={}/usr'.format(
-                    self.args.install_destdir),
+                    self.install_toolchain_path()),
                 '-D', 'CMAKE_MAKE_PROGRAM={}'.format(self.toolchain.ninja),
                 '-D', 'CMAKE_Swift_COMPILER={}'.format(swiftc),
                 '-B', self.build_dir,

utils/swift_build_support/swift_build_support/products/tensorflow.py

@@ -53,7 +53,7 @@ def build(self, host_target):
                 '-G', 'Ninja',
                 '-D', 'BUILD_SHARED_LIBS=YES',
                 '-D', 'CMAKE_INSTALL_PREFIX={}/usr'.format(
-                    self.args.install_destdir),
+                    self.install_toolchain_path()),
                 '-D', 'CMAKE_MAKE_PROGRAM={}'.format(self.toolchain.ninja),
                 '-D', 'CMAKE_Swift_COMPILER={}'.format(swiftc),
                 '-B', self.build_dir,