The net6.0 test build is coupled to the net462 Mono AOT step

[parth-io]

Problem

CI runs the default net6.0 test flow, but the build fails on Linux/macOS with
mono: command not found, though net6.0 doesn't need net462 or Mono.

Cause: build.proj's Test → Build compiles the whole solution, which
includes the net462 Clojure.Compile project even if the CI is testing net6.0 which
doesn't need Mono. Its post-build target
(Clojure.Compile.csproj) AOT-compiles the standard library by running the
net462 compiler, via Mono for Unix:

<TargetCmdline Condition=" '$(OS)' == 'Unix' ">mono $(TargetPath)</TargetCmdline>

So net6.0 CI drags in Mono purely as a build side-effect, to produce net462
artifacts it never loads (net6.0 loads the stdlib from source / embedded resources
at startup; only net462 needs the AOT + ILMerge).

Current stopgap

.github/workflows/tests.yml now installs Mono on the Linux/macOS too.

Why decouple

• Removes an irrelevant dependency
• Clearer build graph

Proposed approach

When TestTargetFramework != net462, don't build Clojure.Compile /
Clojure.Main461 (or skip the Mono AOT post-build).