gh-36900: Fix build of optional Cython modules broken in #29386

The Cython modules depending on optional packages (such as
sage.libs.coxeter3) were not being built after #29386. This showed up in
the CI as numerous doctest failures; as reported in
#36775 (comment)

Fixed here by avoiding a clash of environment variables that prevented
the function `is_package_installed_and_updated` from functioning during
the build of sagelib.

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URL: #36900
Reported by: Matthias Köppe
Reviewer(s): Dima Pasechnik

Author: Release Manager

build/pkgs/configure/checksums.ini

@@ -1,4 +1,4 @@
 tarball=configure-VERSION.tar.gz
-sha1=7f2fe8137f5998559f8d3a0a7f965adf6d5ebc78
-md5=586738771174a2d26d29ce7ba571a95d
-cksum=1156937644
+sha1=8b7ffb6e3984e747b42d30b458220fe60c8ea50a
+md5=eee78cc60e5bb6176640574eb28c17cb
+cksum=1389896899

build/pkgs/configure/package-version.txt

@@ -1 +1 @@
-2395f7bb45fd7da537a953055df5e0a70dd96c6a
+4a970443cd567705f67fcafa7ce1173c45361e3a

src/bin/sage-env

@@ -228,7 +228,7 @@ fi
 # depending on SAGE_ROOT and SAGE_LOCAL which are already defined.
 if [ -n "$SAGE_LOCAL" ]; then
     export SAGE_SHARE="$SAGE_LOCAL/share"
-    export SAGE_SPKG_INST="$SAGE_LOCAL/var/lib/sage/installed"
+    export SAGE_SPKG_INST="$SAGE_LOCAL/var/lib/sage/installed"  # deprecated
 fi
 if [ -n "$SAGE_SHARE" ]; then
     export SAGE_DOC="$SAGE_SHARE/doc/sage"

src/sage/env.py

@@ -177,7 +177,8 @@ def var(key: str, *fallbacks: Optional[str], force: bool = False) -> Optional[st
 SAGE_LOCAL = var("SAGE_LOCAL", SAGE_VENV)
 SAGE_SHARE = var("SAGE_SHARE", join(SAGE_LOCAL, "share"))
 SAGE_DOC = var("SAGE_DOC", join(SAGE_SHARE, "doc", "sage"))
-SAGE_SPKG_INST = var("SAGE_SPKG_INST", join(SAGE_LOCAL, "var", "lib", "sage", "installed"))
+SAGE_LOCAL_SPKG_INST = var("SAGE_LOCAL_SPKG_INST", join(SAGE_LOCAL, "var", "lib", "sage", "installed"))
+SAGE_SPKG_INST = var("SAGE_SPKG_INST", join(SAGE_LOCAL, "var", "lib", "sage", "installed"))  # deprecated
 
 # source tree of the Sage distribution
 SAGE_ROOT = var("SAGE_ROOT")  # no fallback for SAGE_ROOT

src/sage/misc/package.py

@@ -390,7 +390,7 @@ def _spkg_inst_dirs():
     """
     Generator for the installation manifest directories as resolved paths.
 
-    It yields first ``SAGE_SPKG_INST``, then ``SAGE_VENV_SPKG_INST``,
+    It yields first ``SAGE_LOCAL_SPKG_INST``, then ``SAGE_VENV_SPKG_INST``,
     if defined; but it both resolve to the same directory, it only yields
     one element.
 
@@ -402,7 +402,7 @@ def _spkg_inst_dirs():
 
     """
     last_inst_dir = None
-    for inst_dir in (sage.env.SAGE_SPKG_INST, sage.env.SAGE_VENV_SPKG_INST):
+    for inst_dir in (sage.env.SAGE_LOCAL_SPKG_INST, sage.env.SAGE_VENV_SPKG_INST):
         if inst_dir:
             inst_dir = Path(inst_dir).resolve()
             if inst_dir.is_dir() and inst_dir != last_inst_dir: