[build-script] Set the TOOLCHAINS variable in the environment.

On Darwin platforms, the Swift build scripts generally use "xcrun" to
locate various tools, and the build-script's darwin-xcrun-toolchain option
is specified to xcrun. However, with the change to use the just-built clang
to build the runtime (PR #6112), there was an issue with the linker. Clang
normally tries to run the linker from the same directory as the clang
binary, but when there isn't one, it falls back on /usr/bin/ld, which is
just an xcrun-like wrapper. Since there is no way to specify an explicit
toolchain option in that case, set TOOLCHAINS in the environment to
make it use the linker from the specified toolchain.
rdar://problem/30709330

utils/build-script

@@ -2173,6 +2173,7 @@ iterations with -O",
 
     # Prepare and validate toolchain
     toolchain = host_toolchain(xcrun_toolchain=args.darwin_xcrun_toolchain)
+    os.environ['TOOLCHAINS'] = args.darwin_xcrun_toolchain
 
     if args.host_cc is not None:
         toolchain.cc = args.host_cc