Fix snapshot-diff detection for extraneous files

[mtzguido]

Plus, add a goodie to the workflow, a new section with a summary of the build failures, obtained by simply grepping for *** and (Error and adding some context lines.

You'll notice a previous build of this PR failed due to a missing menhir in the CI machine. The Output build log summary section contains only this:

#23 33.48 cd /home/opam/FStar/ocaml && { dune clean || true ; }
#23 33.48 rm -rf /home/opam/FStar/ocaml/fstar-lib/generated/*
#23 33.48 rm -rf /home/opam/FStar/ocaml/fstar-lib/dynamic/*
#23 33.48 make[2]: Leaving directory '/home/opam/FStar'
#23 33.48 make dune-bootstrap
#23 33.49 make dune-extract-all
#23 33.49 make -C src/ocaml-output dune-snapshot
#23 33.5[1](https://github.com/FStarLang/FStar/actions/runs/4874710672/jobs/8695985641#step:13:1) Makefile:49: *** Correct version of menhir not found (needs a version newer than 20[16](https://github.com/FStarLang/FStar/actions/runs/4874710672/jobs/8695985641#step:13:17)1115).  Stop.
#23 33.51 make[4]: Entering directory '/home/opam/FStar/src/ocaml-output'
#23 33.51 make[4]: Leaving directory '/home/opam/FStar/src/ocaml-output'
#23 33.51 make[3]: *** [Makefile:46: dune-extract-all] Error 2
#23 33.51 make[2]: *** [Makefile:56: dune-bootstrap] Error 2
#23 33.51 make[1]: *** [Makefile:52: dune-full-bootstrap] Error 2
#23 33.51 make: *** [Makefile:120: ci-utest-prelude] Error 2
#23 33.51 + echo Warm-up failed
#23 33.51 + echo Failure
#23 33.51 Warm-up failed
#23 33.51 + return 1
#23 ERROR: process "/bin/bash --login -c eval $(opam env) && Z3_LICENSE=\"$(opam config expand '%{prefix}%')/.opam-switch/sources/z3.4.8.5/LICENSE.txt\" CI_NO_KARAMEL=$CI_NO_KARAMEL DZOMO_GITHUB_TOKEN=$(sudo cat /run/secrets/DZOMO_GITHUB_TOKEN) .docker/build/build-standalone.sh $CI_TARGET $CI_THREADS $CI_BRANCH" did not complete successfully: exit code: 1
------
 > [stage-0 [18](https://github.com/FStarLang/FStar/actions/runs/4874710672/jobs/8695985641#step:13:19)/[19](https://github.com/FStarLang/FStar/actions/runs/4874710672/jobs/8695985641#step:13:20)] RUN --mount=type=secret,id=DZOMO_GITHUB_TOKEN eval $(opam env) && Z3_LICENSE="$(opam config expand '%{prefix}%')/.opam-switch/sources/z3.4.8.5/LICENSE.txt" CI_NO_KARAMEL= DZOMO_GITHUB_TOKEN=$(sudo cat /run/secrets/DZOMO_GITHUB_TOKEN) .docker/build/build-standalone.sh uregressions 24 guido_ci:
#23 33.51 make[4]: Entering directory '/home/opam/FStar/src/ocaml-output'
#23 33.51 make[4]: Leaving directory '/home/opam/FStar/src/ocaml-output'
#23 33.51 make[3]: *** [Makefile:46: dune-extract-all] Error 2
#23 33.51 make[2]: *** [Makefile:56: dune-bootstrap] Error 2
#23 33.51 make[1]: *** [Makefile:52: dune-full-bootstrap] Error 2
#23 33.51 make: *** [Makefile:1[20](https://github.com/FStarLang/FStar/actions/runs/4874710672/jobs/8695985641#step:13:21): ci-utest-prelude] Error 2
#[23](https://github.com/FStarLang/FStar/actions/runs/4874710672/jobs/8695985641#step:13:24) [33](https://github.com/FStarLang/FStar/actions/runs/4874710672/jobs/8695985641#step:13:34).[51](https://github.com/FStarLang/FStar/actions/runs/4874710672/jobs/8695985641#step:13:52) + echo Warm-up failed
#23 33.51 + echo Failure
#23 33.51 Warm-up failed
#23 33.51 + return 1
------
standalone.Dockerfile:88
--------------------
  86 |     ARG CI_BRANCH=master
  87 |     ARG CI_NO_KARAMEL=
  88 | >>> RUN --mount=type=secret

Adding menhir to the CI machine to fix this now.