build_environment: clean *_ROOT variables (#26474)

Co-authored-by: Harmen Stoppels <harmenstoppels@gmail.com>

lib/spack/spack/build_environment.py

@@ -185,6 +185,13 @@ def clean_environment():
     env.unset('LD_PRELOAD')
     env.unset('DYLD_INSERT_LIBRARIES')
 
+    # Avoid <packagename>_ROOT user variables overriding spack dependencies
+    # https://cmake.org/cmake/help/latest/variable/PackageName_ROOT.html
+    # Spack needs SPACK_ROOT though, so we need to exclude that
+    for varname in os.environ.keys():
+        if varname.endswith('_ROOT') and varname != 'SPACK_ROOT':
+            env.unset(varname)
+
     # On Cray "cluster" systems, unset CRAY_LD_LIBRARY_PATH to avoid
     # interference with Spack dependencies.
     # CNL requires these variables to be set (or at least some of them,