sagemathgh-38304: Fix sagelib install with `configure --disable-edita…

…ble`

    
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As reported by @culler, when Sage is configured with `./configure
--disable-editable` silently fails to install the Sage library in the
venv (including scripts such as `sage-ipython`).

`./sage -pip list | grep sage ` will show no `sagemath-standard`

The build log shows these messages:
```
[spkg-install] Successfully built
sagemath_standard-10.4rc1-cp312-cp312-macosx_14_0_x86_64.whl
[spkg-install] Staged wheel file, staged /doesnotexist/sagelib/spkg-
requirements.txt
[spkg-install] /Users/mkoeppe/s/sage/sage-rebasing/worktree-
pristine/build/bin/sage-dist-helpers: line 367:
/doesnotexist/sagelib/spkg-pipinst: No such file or directory
[spkg-install] /Users/mkoeppe/s/sage/sage-rebasing/worktree-
pristine/build/bin/sage-dist-helpers: line 387:
/doesnotexist/sagelib/spkg-piprm: No such file or directory
Moving package files from temporary location /Users/mkoeppe/s/sage/sage-
rebasing/worktree-pristine/venv-
wheels/var/tmp/sage/build/sagelib-10.4.rc1/inst to
/Users/mkoeppe/s/sage/sage-rebasing/worktree-pristine/venv-wheels
Deleting build directory /Users/mkoeppe/s/sage/sage-rebasing/worktree-
pristine/venv-wheels/var/tmp/sage/build/sagelib-10.4.rc1
Finished installing sagelib-10.4.rc1
```

Fixed here in this PR.

- Also fixes sagemath#38293.
- Also fixes sagemath#38253.

### 📝 Checklist

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- [ ] I have linked a relevant issue or discussion.
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- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

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<!-- - sagemath#12345: short description why this is a dependency -->
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URL: sagemath#38304
Reported by: Matthias Köppe
Reviewer(s):

build/pkgs/sagelib/spkg-install.in

@@ -24,11 +24,10 @@ export SAGE_ROOT=/doesnotexist
 export SAGE_SRC=/doesnotexist
 export SAGE_SRC_ROOT=/doesnotexist
 export SAGE_DOC_SRC=/doesnotexist
-export SAGE_BUILD_DIR=/doesnotexist
 
-# We also poison all directories below SAGE_LOCAL.
+# We also poison all directories below SAGE_LOCAL
+# except for SAGE_SPKG_SCRIPTS, which is needed by sage-dist-helpers
 export SAGE_PKGCONFIG=/doesnotexist
-export SAGE_SPKG_SCRIPTS=/doesnotexist
 export SAGE_SHARE=/doesnotexist
 
 # However, we only unset SAGE_LOCAL and SAGE_PKG_CONFIG_PATH.