meson: Work around Meson install_subdir limitation

When install_subdir encounters a mkosi.tools tree with a /bin to
/usr/bin symlink it fails to copy it because it dereferences but still
treats it like a file.
Work around the Meson bug by excluding the mkosi.tools tree from
installation like mkosi.local is excluded. We anyway don't want the
tools tree end up there.

meson.build

@@ -2777,7 +2777,7 @@ if install_tests
         install_subdir('mkosi',
                        install_dir : testsdir,
                        exclude_files : ['mkosi.local.conf', 'mkosi.key', 'mkosi.crt'],
-                       exclude_directories : ['mkosi.local'])
+                       exclude_directories : ['mkosi.local', 'mkosi.tools'])
 endif
 
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