Unbreak finding gstreamer plugins in $NIX_PROFILES

[bjornfor]

Motivation for this change

Making gst-launch and friends find plugins is a generally useful thing. It used to work but got broken with the multiple-outputs change. This PR fixes it.

Also lower the priority on plugins found in $NIX_PROFILES by using --suffix instead of --prefix. This is the saner priority setting IMHO.

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[mention-bot]

@bjornfor, thanks for your PR! By analyzing the history of the files in this pull request, we identified @ttuegel, @vcunat and @dezgeg to be potential reviewers.

[vcunat]

Maybe a separate .bin output would be useful, as .dev won't be installed into standard environments by default.

[bjornfor]

@dezgeg added the outputBin = "dev" change. Perhaps there was a good reason for using dev instead of bin? @dezgeg, do you expect any breakage if we remove outputBin = "dev" and change --exec-prefix from $dev to $bin?

[vcunat]

He just refactored and fixed some issues; it was @ttuegel who split gstreamer and started moving binaries to $dev 6a64edf.

[vcunat]

Note that the derivation has no $bin ATM.

[ttuegel]

I put the binaries in $dev because their intended audience seems to be developers working on GStreamer-based applications. If they are useful to end-users, then we should move them into $bin.

As for finding plugins in $NIX_PROFILES: do we want the binaries to find plugins there when the library will not? That might make debugging other applications more difficult for users.

[bjornfor]

@ttuegel: Good point. But I don't know when I'll have time to move the lookup from the binaries to the libraries. (It's not high priority for me.)

How about merging this now and improve things more later?

[bjornfor]

I think we should merge as is, so we can backport to stable without risk.

[vcunat]

OK, picked to 16.09 as well. Still, multiple outputs generally behave differently than intended due to the default nix version not containing some relatively old patches #19761 (comment). When/if that gets fixed, people will no longer get the $dev outputs in environments by default...