feat(tvix/eval): implement tvix's user-agent, err, nixVersion

nixpkgs has hardcoded references to Nix versions, we need to provide
it with something that looks like a Nix version while actually being a
Tvix version.

For now, we do this by stealing a trick out of the browser book and
constructing a version that looks like a Nix version to Nix, but like
a Tvix version to people who know what they are looking for.

Nevermind that we don't actually have any kind of versioning for
Tvix (yet?), other than depot revisions.

Change-Id: I7ce8079dd8164a2079891d38e707f09a45f0bbc1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6858
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/tests/tvix_tests/eval-okay-nix-version-cmp.exp

@@ -0,0 +1 @@
+-1

tvix/eval/src/tests/tvix_tests/eval-okay-nix-version-cmp.nix

@@ -0,0 +1,5 @@
+# nixpkgs checks against the `builtins.nixVersion` and fails if it
+# doesn't like what it sees. To work around this we have a "user-agent
+# style" version (see cl/6858) that ensures compatibility.
+
+builtins.compareVersions "2.3" builtins.nixVersion