spack-environment: use %gcc for py-tensorflow, revert all:require to '%gcc'

The previous fix (any_of: ['%gcc', '%clang'] in all:require) allowed
py-tensorflow to concretize with %clang but caused ~24 packages to be
duplicated (once compiled with %gcc, once with %clang).

Root cause analysis: py-tensorflow@2.20's own compiler constraints only
require gcc@12+ and clang@16+; both gcc@13.3.0 and clang@17.0.6 are
valid. The '%clang' requirement in packages.yaml was a preference, not
a technical necessity.

In Spack v1.1+, 'all: require: %gcc' applies to ALL packages without
exception and cannot be overridden by package-specific require entries
(both constraints apply simultaneously). Removing the '%clang' preference
from py-tensorflow:require eliminates the compiler conflict entirely,
and the all:require:'%gcc' preference guides py-tensorflow to use %gcc
consistently with all other packages.

Also update the misleading comment that claimed 'require for all is
overwritten by package-specific entries' — this was incorrect for
Spack v1.1+.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: wdconinc

spack-environment/packages.yaml

@@ -11,17 +11,17 @@
 #
 packages:
   all:
-    # duplication between the prefer and require sections is intentional
-    # since 'require' for 'all' is overwritten by package-specific entries,
-    # and 'prefer' at least retains some (less strict) preference in that case
+    # note: in Spack v1.1+, 'require' for 'all' is NOT overwritten by package-specific
+    # entries; both constraints apply simultaneously. 'prefer' for 'all' retains a
+    # weaker preference that is superseded by package-specific requirements.
     prefer:
     - '%gcc'
     # FIXME any ipo preference breaks llvm external reuse
     #- +ipo
     - build_system=cmake
     - build_type=Release
     require:
-    - any_of: ['%gcc', '%clang']  # allow packages with specific compiler requirements (e.g. py-tensorflow)
+    - '%gcc'
     # FIXME any ipo requirement breaks llvm external reuse
     #- any_of: [+ipo, '@:']
     - any_of: [build_system=cmake, '@:']