Fix ci-bonsai-choco: choco_release.py still targets the old choco/blenderbim path

The choco dir was renamed from choco/blenderbim to choco/bonsai back in
2024 (Rename choco dir), but choco_release.py's BLENDERBIM_DIR constant
was never updated, so the daily choco release job crashes immediately
with FileNotFoundError trying to os.chdir into the now nonexistent
choco/blenderbim directory.

Release tags also moved from a bare blenderbim-YYMMDD scheme to
bonsai-X.Y.Z-alphaYYMMDDHHMM, so the tag-prefix strip used to build the
package version still looked for the old "blenderbim-" prefix and left
it untouched, embedding the raw tag (including the already-present
"-alpha" segment) into the nuspec version field, which the template
then doubled up with its own "-alpha" suffix, producing an invalid
NuGet version string. Both are fixed together since the second bug
would otherwise surface as soon as the first one is unblocked.

The pre-commit black hook also reformatted pre-existing whitespace
drift in choco_release.py (this file sits outside CI's lint scope, so
it had never been auto-formatted before); that reformatting is
incidental to satisfying the local hook, not part of the fix itself.

Generated with the assistance of an AI coding tool.

choco/bonsai/blenderbim.nuspec

@@ -3,7 +3,7 @@
 <package xmlns="http://schemas.microsoft.com/packaging/2015/06/nuspec.xsd">
   <metadata>
     <id>blenderbim-nightly</id>
-    <version>blenderbim_build_version-alpha</version>
+    <version>blenderbim_build_version</version>
     <packageSourceUrl>https://github.com/IfcOpenShell/IfcOpenShell</packageSourceUrl>
     <owners>fbpyr</owners>
     <!-- == SOFTWARE SPECIFIC SECTION == -->