Make sure version number gets updated without having to run dune clean

[Alasdair]

Adds (deps (universe)) to the action that builds manifest.ml so it is always rebuilt (unless it was generated from manifest.ml.in by opam, as the action still has fallback mode).

Fixes #204, and based on PR #863

[Alasdair]

Tested by making a dummy commit, and using git checkout between that commit and HEAD~1 followed by make install causes the hash in sail --version to update immediately.

For the opam install functionality opam config subst src/bin/manifest.ml followed by dune build --release --verbose shows that the sail_manifest binary is not run as expected, and sail --version shows the opam version number.

[github-actions]

Test Results

   12 files  ±0     24 suites  ±0   0s ⏱️ ±0s
  741 tests ±0    741 ✅ ±0  0 💤 ±0  0 ❌ ±0 
2 476 runs  ±0  2 476 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 35bfd4f. ± Comparison against base commit 18b0aec.

♻️ This comment has been updated with latest results.