CMakeLists: Fix deduplication trap with --preload-file

[fwcd]

CMake applies a form of deduplication that, when specifying

target_link_options(mixxx-lib PUBLIC --preload-file a)
target_link_options(mixxx-lib PUBLIC --preload-file b)

would only result in

--preload-file a b

because it considers --preload-file to be a flag that was already set. This leads to very confusing error messages, because the Emscripten compiler would consider b to be a positional argument (i.e. a source file) rather than a value to --preload-file.

While this currently isn't an issue, because we only set this flag once, the current syntax is misleading and could easily lead to issues if we e.g. split up the resources as proposed in the adjacent TODO comment. For this reason, this PR migrates to the --preload-file=... syntax to avoid this.

Side note: Another way to do this would be to use "SHELL:--preload-file ...", but then we would likely have to deal with escaping/quoting the path ourselves to make it robust against spaces. For details, see this upstream CMake issue and this CMake PR.

[JoergAtGithub]

LGTM! Local build works as before! Thank you!