Introduce a SPACK_PYTHON environment variable (#21222)

The SPACK_PYTHON environment variable can be set to a python interpreter to be
used by the spack command.  This allows the spack command itself to use a
consistent and separate interpreter from whatever python might be used for package
building.

share/spack/setup-env.csh

@@ -58,6 +58,18 @@ alias spack          'set _sp_args = (\!*); source $_spack_share_dir/csh/spack.c
 alias spacktivate    'spack env activate'
 alias _spack_pathadd 'set _pa_args = (\!*) && source $_spack_share_dir/csh/pathadd.csh'
 
+# Identify and lock the python interpreter
+if (! $?SPACK_PYTHON) then
+    setenv SPACK_PYTHON ""
+endif
+foreach cmd ("$SPACK_PYTHON" python3 python python2)
+    command -v "$cmd" >& /dev/null
+    if ($status == 0) then
+        setenv SPACK_PYTHON `command -v "$cmd"`
+        break
+    endif
+end
+
 # Set variables needed by this script
 _spack_pathadd PATH "$SPACK_ROOT/bin"
 eval `spack --print-shell-vars csh`

share/spack/setup-env.fish

@@ -253,7 +253,7 @@ function match_flag -d "checks all combinations of flags ocurring inside of a st
     set -l _a (string sub -s 2 (string trim "x$argv[1]"))
     set -l _b (string sub -s 2 (string trim "x$argv[2]"))
 
-    if test -z "$_a" || test -z "$_b"
+    if test -z "$_a" or test -z "$_b"
         return 0
     end
 
@@ -667,6 +667,19 @@ set -l sp_source_file (status -f)  # name of current file
 
 
 
+#
+# Identify and lock the python interpreter
+#
+for cmd in "$SPACK_PYTHON" python3 python python2
+    set -l _sp_python (command -s "$cmd")
+    if test $status -eq 0
+        set -x SPACK_PYTHON $_sp_python
+        break
+    end
+end
+
+
+
 #
 # Find root directory and add bin to path.
 #

share/spack/setup-env.sh

@@ -326,6 +326,14 @@ if [ "$_sp_shell" = bash ]; then
     export -f _spack_shell_wrapper
 fi
 
+# Identify and lock the python interpreter
+for cmd in "${SPACK_PYTHON:-}" python3 python python2; do
+    if command -v > /dev/null "$cmd"; then
+        export SPACK_PYTHON="$(command -v "$cmd")"
+        break
+    fi
+done
+
 #
 # make available environment-modules
 #