static btrfs snapshot paths

i don't know the implications of this change, except for maybe
concurrent borgmatic processes

borgmatic/hooks/data_source/btrfs.py

@@ -180,7 +180,7 @@ def get_subvolumes(btrfs_command, patterns):
     return tuple(sorted(subvolumes, key=lambda subvolume: subvolume.path))
 
 
-BORGMATIC_SNAPSHOT_PREFIX = '.borgmatic-snapshot-'
+BORGMATIC_SNAPSHOT_PREFIX = '.borgmatic-snapshot'
 
 
 def make_snapshot_path(subvolume_path):
@@ -189,7 +189,7 @@ def make_snapshot_path(subvolume_path):
     '''
     return os.path.join(
         subvolume_path,
-        f'{BORGMATIC_SNAPSHOT_PREFIX}{os.getpid()}',
+        f'{BORGMATIC_SNAPSHOT_PREFIX}',
         # Included so that the snapshot ends up in the Borg archive at the "original" subvolume path.
     ) + subvolume_path.rstrip(os.path.sep)
 
@@ -244,7 +244,7 @@ def make_borg_snapshot_pattern(subvolume_path, pattern):
 
     rewritten_path = initial_caret + os.path.join(
         subvolume_path,
-        f'{BORGMATIC_SNAPSHOT_PREFIX}{os.getpid()}',
+        f'{BORGMATIC_SNAPSHOT_PREFIX}',
         # Use the Borg 1.4+ "slashdot" hack to prevent the snapshot path prefix from getting
         # included in the archive—but only if there's not already a slashdot hack present in the
         # pattern.