Wrong environment passed to happy build tool

**Describe the bug**

This was initially reported in https://github.com/haskell/cabal/issues/10072#issuecomment-2513888478 but I'm not sure it's the same issue so I don't want to hijack Sebastian's ticket.

1. I have happy 2.0.2 globally installed (`/usr/bin/happy`)
2. For some reason Cabal puts happy 2.1.3 in the build plan and builds it
3. But then my globally installed happy 2.0.2 is called with the environment variable `happy_lib_datadir` set to `/path/to/happy-lib-2.1.3/data`, hence it uses the wrong `HappyTemplate.hs` and the generated code doesn't compile.

**To Reproduce**

Steps to reproduce the behavior:

In GHC's source tree:

```
# Preparing source files...
mkdir -p _build/stage0/src/
cp -rf ./libraries _build/stage0/src/
cp -rf ./compiler _build/stage0/src/libraries/ghc
cp -rf ./ghc _build/stage0/src/ghc-bin
cp -rf ./utils _build/stage0/src/

## Substituting variables
cp _build/stage0/src/ghc-bin/ghc-bin.cabal{.in,}
cp _build/stage0/src/libraries/ghc/ghc.cabal{.in,}
cp _build/stage0/src/libraries/ghc/GHC/CmmToLlvm/Version/Bounds.hs{.in,}
cp _build/stage0/src/libraries/ghc-boot/ghc-boot.cabal{.in,}
cp _build/stage0/src/libraries/ghc-boot-th/ghc-boot-th.cabal{.in,}
cp _build/stage0/src/libraries/ghc-heap/ghc-heap.cabal{.in,}
cp _build/stage0/src/libraries/ghci/ghci.cabal{.in,}
cp _build/stage0/src/utils/ghc-pkg/ghc-pkg.cabal{.in,}

sed -i 's/@ProjectVersion@/9.13/' _build/stage0/src/ghc-bin/ghc-bin.cabal
sed -i 's/@ProjectVersionMunged@/9.13/' _build/stage0/src/ghc-bin/ghc-bin.cabal
sed -i 's/@ProjectVersion@/9.13/' _build/stage0/src/libraries/ghc/ghc.cabal
sed -i 's/@ProjectVersionMunged@/9.13/' _build/stage0/src/libraries/ghc/ghc.cabal
sed -i 's/@ProjectVersion@/9.13/' _build/stage0/src/libraries/ghc-boot/ghc-boot.cabal
sed -i 's/@ProjectVersionMunged@/9.13/' _build/stage0/src/libraries/ghc-boot/ghc-boot.cabal
sed -i 's/@ProjectVersion@/9.13/' _build/stage0/src/libraries/ghc-boot-th/ghc-boot-th.cabal
sed -i 's/@ProjectVersionMunged@/9.13/' _build/stage0/src/libraries/ghc-boot-th/ghc-boot-th.cabal
sed -i 's/@Suffix@//' _build/stage0/src/libraries/ghc-boot-th/ghc-boot-th.cabal
sed -i 's/@SourceRoot@/./' _build/stage0/src/libraries/ghc-boot-th/ghc-boot-th.cabal
sed -i 's/@ProjectVersion@/9.13/' _build/stage0/src/libraries/ghc-heap/ghc-heap.cabal
sed -i 's/@ProjectVersionMunged@/9.13/' _build/stage0/src/libraries/ghc-heap/ghc-heap.cabal
sed -i 's/@ProjectVersionForLib@/9.13/' _build/stage0/src/libraries/ghc-heap/ghc-heap.cabal
sed -i 's/@ProjectVersion@/9.13/' _build/stage0/src/libraries/ghci/ghci.cabal
sed -i 's/@ProjectVersionMunged@/9.13/' _build/stage0/src/libraries/ghci/ghci.cabal
sed -i 's/@ProjectVersionForLib@/9.13/' _build/stage0/src/libraries/ghci/ghci.cabal
sed -i 's/@ProjectVersion@/9.13/' _build/stage0/src/utils/ghc-pkg/ghc-pkg.cabal
sed -i 's/@ProjectVersionMunged@/9.13/' _build/stage0/src/utils/ghc-pkg/ghc-pkg.cabal
sed -i 's/@ProjectVersionForLib@/9.13/' _build/stage0/src/utils/ghc-pkg/ghc-pkg.cabal

sed -i 's/@LlvmMinVersion@/13/' _build/stage0/src/libraries/ghc/GHC/CmmToLlvm/Version/Bounds.hs
sed -i 's/@LlvmMaxVersion@/20/' _build/stage0/src/libraries/ghc/GHC/CmmToLlvm/Version/Bounds.hs

# Building...
mkdir -p _build/stage0/cabal/
mkdir -p _build/stage0/bin/

$(CABAL) build --project-file=cabal.project-stage0 \
ghc-bin:ghc ghc-pkg:ghc-pkg genprimopcode:genprimopcode deriveConstants:deriveConstants \
-j --builddir=_build/stage0/cabal/
```

```
-- cabal.project-stage0
packages:
./_build/stage0/src/ghc-bin/
./_build/stage0/src/libraries/ghc
./_build/stage0/src/libraries/directory/
./_build/stage0/src/libraries/file-io/
./_build/stage0/src/libraries/filepath/
./_build/stage0/src/libraries/ghc-platform/
./_build/stage0/src/libraries/ghc-boot/
./_build/stage0/src/libraries/ghc-boot-th/
./_build/stage0/src/libraries/ghc-heap
./_build/stage0/src/libraries/ghci
./_build/stage0/src/libraries/os-string/
./_build/stage0/src/libraries/process/
./_build/stage0/src/libraries/semaphore-compat
./_build/stage0/src/libraries/time
./_build/stage0/src/libraries/unix/
./_build/stage0/src/libraries/Win32/
./_build/stage0/src/utils/ghc-pkg
./_build/stage0/src/utils/hsc2hs
./_build/stage0/src/utils/unlit
./_build/stage0/src/utils/genprimopcode/
./_build/stage0/src/utils/deriveConstants/

benchmarks: False
tests: False
allow-boot-library-installs: True

program-options
ghc-options: -fhide-source-paths -j

package *
library-vanilla: True
shared: False
executable-profiling: False
executable-dynamic: False
executable-static: False
ghc-options: -fhide-source-paths -j

package ghc-boot-th
flags: +bootstrap

-- package genprimopcode
-- flags: -build-tool-depends

allow-newer: ghc-boot-th
```

**Expected behavior**

It should build without failing.

**System information**
- Operating system: linux
- Fails with cabal-install 3.14.1.0 and HEAD. Works with cabal-install 3.12.1.0

**Additional context**

I've bisected and the commit introducing the issue is 8bdda9c0b6231f05eda3e06f677ec5b71bfbb7cf (cc @sheaf).

Start by reviewing commit 8bdda9c0b6231f05eda3e06f677ec5b71bfbb7cf and the cabal.project-stage0 configuration described in the report. Reproduce the GHC stage0 build with cabal-install 3.14.1.0 or HEAD, compare it with 3.12.1.0, and verify that the build completes without invoking the wrong Happy environment.

Written by the indexing model from the issue text.