packaging: explain why we purge while building wheels

We also make is a bit narrower so that it is less surprising.

contrib/packaging/build-linux-wheels.sh

@@ -1,44 +1,48 b''
 #!/bin/bash
 
 # Intended to run within docker using image:
 #
 #  registry.heptapod.net/mercurial/ci-images/core-wheel-x86_64-c:v3.0
 #
 # we might want to factor most of this with the associated mercurial-core CI
 # definition. (i.e. move this script into a place where the CI can directly call it for its purpose)
 
 set -e -x
 
 PYTHON_TARGETS="cp38-cp38 cp39-cp39 cp310-cp310 cp311-cp311 cp312-cp312 cp313-cp313"
 
 export MERCURIAL_SETUP_FORCE_TRANSLATIONS=1
 
 # We need to copy the repository to ensure:
 # (1) we don't wrongly write roots files in the repository (or any other wrong
 #     users)
 # (2) we don't reuse pre-compiled extension built outside for manylinux and
 #     therefor not compatible.
 cp -r /src/ /tmp/src/
 cd /tmp/src/
-hg purge --all --no-confirm
+# clear potentially cached artifact from the host
+# (we could narrow this purge probably)
+hg purge \
+    --ignored \
+    --no-confirm
 
 export HGRCPATH=/tmp/build-config.rc
 cat << EOF > $HGRCPATH
 [trusted]
 users=*
 groups=*
 EOF
 
 for py in $PYTHON_TARGETS; do
     echo 'build wheel for' $py
     # cleanup any previous wheel
     tmp_wd="/tmp/wheels/$py/repaired"
     rm -rf $tmp_wd
     mkdir -p $tmp_wd
     # build a new wheel
     contrib/build-one-linux-wheel.sh $py $tmp_wd
     # fix the owner back to the repository owner
     chown `stat /src/ -c %u:%g` $tmp_wd/*.whl
     mv $tmp_wd/*.whl /src/dist
 done