build-script: validate arguments before running sccache (#61375)

When a user passes `--sccache` flag when `sccache` is not installed, we still attempt to make a shell invocation to `sccache`, which leads to an inscrutable error:

```
Traceback (most recent call last):
  File "/root/swift-source/swift/utils/build-script", line 789, in <module>
    exit_code = main()
  File "/root/swift-source/swift/utils/build-script", line 784, in main
    return main_normal()
  File "/root/swift-source/swift/utils/build-script", line 687, in main_normal
    shell.capture([toolchain.sccache, "--show-stats"])
  File "/root/swift-source/swift/utils/swift_build_support/swift_build_support/shell.py", line 133, in capture
    return subprocess.check_output(command, env=_env, stderr=stderr,
  File "/usr/lib/python3.10/subprocess.py", line 420, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  File "/usr/lib/python3.10/subprocess.py", line 501, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/usr/lib/python3.10/subprocess.py", line 969, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib/python3.10/subprocess.py", line 1720, in _execute_child
    and os.path.dirname(executable)
  File "/usr/lib/python3.10/posixpath.py", line 152, in dirname
    p = os.fspath(p)
TypeError: expected str, bytes or os.PathLike object, not NoneType
```

`validate_arguments` should be called before `shell.capture([toolchain.sccache, "--show-stats"])` so that a more meaningful error message is shown to the user, asking them to install `sccache`.

Author: MaxDesiatov

utils/build-script

@@ -679,12 +679,6 @@ def main_normal():
         toolchain.libtool = args.host_libtool
     if args.cmake is not None:
         toolchain.cmake = args.cmake
-    if args.sccache:
-        print("Ensuring the sccache server is running...")
-        # Use --show-stats to ensure the server is started, because using
-        # --start-server will fail if the server is already running.
-        # Capture the output because we don't want to see the stats.
-        shell.capture([toolchain.sccache, "--show-stats"])
 
     cmake = CMake(args=args, toolchain=toolchain)
     # Check the CMake version is sufficient on Linux and build from source
@@ -700,6 +694,13 @@ def main_normal():
     # Validate the arguments.
     validate_arguments(toolchain, args)
 
+    if args.sccache:
+        print("Ensuring the sccache server is running...")
+        # Use --show-stats to ensure the server is started, because using
+        # --start-server will fail if the server is already running.
+        # Capture the output because we don't want to see the stats.
+        shell.capture([toolchain.sccache, "--show-stats"])
+
     # Create the build script invocation.
     invocation = build_script_invocation.BuildScriptInvocation(toolchain, args)