Add $date option to the list of config variables (#33875)

I'm finding I often want the date in my paths and it would be nice if spack had a config variable for this.

Co-authored-by: Tamara Dahlgren <35777542+tldahlgren@users.noreply.github.com>

lib/spack/docs/configuration.rst

@@ -416,6 +416,8 @@ Spack understands several special variables. These are:
   ArchSpec. E.g. ``skylake`` or ``neoverse-n1``.
 * ``$target_family``. The target family for the current host, as
   detected by ArchSpec. E.g. ``x86_64`` or ``aarch64``.
+* ``$date``: the current date in the format YYYY-MM-DD
+
 
 Note that, as with shell variables, you can write these as ``$varname``
 or with braces to distinguish the variable from surrounding characters:

lib/spack/spack/test/config.py

@@ -8,6 +8,7 @@
 import os
 import sys
 import tempfile
+from datetime import date
 
 import pytest
 from six import StringIO
@@ -443,6 +444,13 @@ def test_substitute_tempdir(mock_low_high_config):
     )
 
 
+def test_substitute_date(mock_low_high_config):
+    test_path = os.path.join("hello", "world", "on", "$date")
+    new_path = spack_path.canonicalize_path(test_path)
+    assert "$date" in test_path
+    assert date.today().strftime("%Y-%m-%d") in new_path
+
+
 PAD_STRING = spack.util.path.SPACK_PATH_PADDING_CHARS
 MAX_PATH_LEN = spack.util.path.get_system_path_max()
 MAX_PADDED_LEN = MAX_PATH_LEN - spack.util.path.SPACK_MAX_INSTALL_PATH_LENGTH

lib/spack/spack/util/path.py

@@ -14,6 +14,7 @@
 import subprocess
 import sys
 import tempfile
+from datetime import date
 
 from six.moves.urllib.parse import urlparse
 
@@ -70,6 +71,7 @@ def replacements():
         "os": str(arch.os),
         "target": str(arch.target),
         "target_family": str(arch.target.microarchitecture.family),
+        "date": date.today().strftime("%Y-%m-%d"),
     }
 
 
@@ -285,6 +287,7 @@ def substitute_config_variables(path):
     - $operating_system  The OS of the current system
     - $target            The ISA target detected for the system
     - $target_family     The family of the target detected for the system
+    - $date              The current date (YYYY-MM-DD)
 
     These are substituted case-insensitively into the path, and users can
     use either ``$var`` or ``${var}`` syntax for the variables. $env is only