swift_build_support: fix shell.run bytes printing (#68373)

* swift_build_support: fix `shell.run` bytes printing

`output.splitlines()` returns a value of `bytes` type, not `str`, which leads to `b` prefixed to it when printing.

This led to broken output when running `update-checkout`. Before:

```
[swiftpm]                               b"Already on 'main'"
```

After:

```
[swiftpm]                               Already on 'main'
```

* swift_build_support/shell.py: use `errors='replace'` in `decode`

Author: MaxDesiatov

utils/swift_build_support/swift_build_support/shell.py

@@ -242,7 +242,7 @@ def run(*args, **kwargs):
         _echo_command(dry_run, *args, env=env, prompt="{0}+ ".format(prefix))
         if output:
             for line in output.splitlines():
-                print("{0}{1}".format(prefix, line))
+                print("{0}{1}".format(prefix, line.decode('utf-8', errors='replace')))
         sys.stdout.flush()
         sys.stderr.flush()
     if lock: