restrict alex < 3.5.2.0, more ghc-9.8.4 fixes and ghc-9.12.1 as a build compiler

[shayne-fletcher]

(commit 245912d): alex-3.5.2.0 was released to hackage on 2024-12-30. it produces errors like this

compiler/GHC/Parser/Lexer.x:3471:12: error: [GHC-31744]
    Duplicate INLINE pragmas for ‘alexScanUser’
    at /home/runner/work/ghc-lib/ghc-lib/dist-newstyle/build/x86_64-linux/ghc-9.10.1/ghc-lib-parser-0.20241231/build/GHC/Parser/Lexer.hs:1383:12-23
       compiler/GHC/Parser/Lexer.x:3471:12-23
     |
3471 | -- in this file, alexScanUser gets broken out into a separate function and
     |            ^^^^^^^^^^^^

on flavors >= ghc-9.12.1 e.g. https://github.com/digital-asset/ghc-lib/actions/runs/12553438884. verified that this is a problem for GHC too. see haskell/alex#266

(commit 81196e9): two more fixes relating to 9.8.4 as a build compiler

• hide os-string during calculating parser module deps in ghc-lib-gen to avoid ambiguity with filepath
• avoid link errors resulting from duplicate symbol definitions from 'genSym.c'

(commit 27e2421): add an exclusion to ghc-lib's hs-src dirs

(commit 29ca22c): haskell-hvr/cryptohash-sha256#25 got closed enabling us to remove the extra hadrian cabal flags from 'Ghclibgen.hs' and ghcup now serves ghc-9.12.1 (since 2024-12-15 looks like) so, move ghc-lib-ghc-HEAD-ghc-9.10.1.yml to ghc-lib-ghc-HEAD-ghc-9.12.1.yml, ghc-lib-ghc-master-ghc-9.10.1.yml to ghc-lib-ghc-master-ghc-9.12.1.yml (updating the build compiler to ghc-9.12.1 accordingly) and update the README build badge (after landing this it's probably necessary to modify the repository "required test" settings).