mdist: fix error when running tests with a cross file

Since commit 1420d0dace we use coredata's
cmd_line.txt handler to get the right setup arguments. But there's a bug
in that -- it mishandles cross/native files, producing invalid
descriptions of the command line. The only other place this was used,
though, is when generating meson-log.txt.

Fix it to produce correctly formatted arguments.

Fixes #10980

mesonbuild/coredata.py

@@ -1035,9 +1035,9 @@ def update_cmd_line_file(build_dir: str, options: argparse.Namespace):
 def format_cmd_line_options(options: argparse.Namespace) -> str:
     cmdline = ['-D{}={}'.format(str(k), v) for k, v in options.cmd_line_options.items()]
     if options.cross_file:
-        cmdline += [f'--cross-file {f}' for f in options.cross_file]
+        cmdline += [f'--cross-file={f}' for f in options.cross_file]
     if options.native_file:
-        cmdline += [f'--native-file {f}' for f in options.native_file]
+        cmdline += [f'--native-file={f}' for f in options.native_file]
     return ' '.join([shlex.quote(x) for x in cmdline])
 
 def major_versions_differ(v1: str, v2: str) -> bool: