feat(base): cache uv directory

[carlcsaposs-canonical]

Based on #394

Fixes canonical/charmcraft#2461

uv's cache directory is located in the same parent directory as pip's cache directory

craft-providers/craft_providers/base.py

Lines 831 to 832 in b5b4a45

	guest_cache_proc = executor.execute_run(
	["bash", "-c", "echo -n ${XDG_CACHE_HOME:-${HOME}/.cache}"],

https://docs.astral.sh/uv/reference/storage/#cache-directory
https://pip.pypa.io/en/stable/topics/caching/#default-paths

[carlcsaposs-canonical]

The test failures appear to be unrelated to the changes in this PR (I do not have access to retrigger the tests)

---

I built charmcraft using this branch of craft-providers & the uv caching appears to be working

ubuntu@dualla:~$ git clone https://github.com/carlcsaposs-canonical/charmcraft.git
Cloning into 'charmcraft'...
remote: Enumerating objects: 21395, done.
remote: Counting objects: 100% (707/707), done.
remote: Compressing objects: 100% (503/503), done.
remote: Total 21395 (delta 501), reused 205 (delta 204), pack-reused 20688 (from 4)
Receiving objects: 100% (21395/21395), 8.35 MiB | 9.48 MiB/s, done.
Resolving deltas: 100% (15338/15338), done.
ubuntu@dualla:~$ cd charmcraft/
ubuntu@dualla:~/charmcraft$ git checkout test-uv-cache 
branch 'test-uv-cache' set up to track 'origin/test-uv-cache'.
Switched to a new branch 'test-uv-cache'
ubuntu@dualla:~/charmcraft$ sudo snap install snapcraft --classic
2026-02-09T09:12:57Z INFO Waiting for automatic snapd restart...
snapcraft 8.13.2 from Canonical✓ installed
ubuntu@dualla:~/charmcraft$ snapcraft pack -v
[...]
snapcraft internal error: 1 validation error for Project
version
  Value should have at most 32 items after validation, not 33 [type=too_long, input_value='0.0.post3261+g0ddb3bcba.d20260209', input_type=str]
    For further information visit https://errors.pydantic.dev/2.12/v/too_long
Failed to run snapcraft in instance
Full execution log: '/home/ubuntu/.local/state/snapcraft/log/snapcraft-20260209-091339.692530.log'
ubuntu@dualla:~/charmcraft$ nano snap/snapcraft.yaml 
ubuntu@dualla:~/charmcraft$ git diff
diff --git a/snap/snapcraft.yaml b/snap/snapcraft.yaml
index 7219dcbe..ab9f5f6b 100644
--- a/snap/snapcraft.yaml
+++ b/snap/snapcraft.yaml
@@ -150,7 +150,7 @@ parts:
       sed -i 's|#!/bin/sh|#!/snap/charmcraft/current/bin/python3|' $CRAFT_PART_INSTALL/bin/craftctl
 
       version="$(uv pip show charmcraft | grep "Version:" | cut -d' ' -f2)"
-      /snap/snapcraft/current/libexec/snapcraft/craftctl set version="${version}"
+      /snap/snapcraft/current/libexec/snapcraft/craftctl set version="0.0" #"${version}"
       [ -n "$(echo $version | grep "post")" ] && grade=devel || grade=stable
       /snap/snapcraft/current/libexec/snapcraft/craftctl set grade="${grade}"
 
ubuntu@dualla:~/charmcraft$ snapcraft pack -v
[...]
Packed charmcraft_0.0_arm64.snap
ubuntu@dualla:~/charmcraft$ sudo snap install ./charmcraft_0.0_arm64.snap --dangerous --classic
charmcraft 0.0 installed
ubuntu@dualla:~/charmcraft$ cd
ubuntu@dualla:~$ mkdir foo
ubuntu@dualla:~$ cd foo
ubuntu@dualla:~/foo$ charmcraft init
Charmed operator package file and directory tree initialised.

Now edit the following package files to provide fundamental charm metadata
and other information:

charmcraft.yaml
src/charm.py
src/foo.py
README.md

To manage your charm's dependencies, use uv.

To migrate from the Charm plugin to the uv plugin, see:
https://documentation.ubuntu.com/charmcraft/stable/howto/migrate-plugins/charm-to-uv/

ubuntu@dualla:~/foo$ charmcraft pack
Packed foo_arm64.charm
ubuntu@dualla:~/foo$ ls ~/snap/charmcraft/common/cache/charmcraft/charmcraft-buildd-base-v7.1/BuilddBaseAlias.NOBLE/
pip  uv
ubuntu@dualla:~/foo$ ls ~/snap/charmcraft/common/cache/charmcraft/charmcraft-buildd-base-v7.1/BuilddBaseAlias.NOBLE/uv/
CACHEDIR.TAG  archive-v0  interpreter-v4  sdists-v9  wheels-v5

[bepri]

Lint error is fixed in #910

[bepri]

Also, the test failures involving "tracemalloc" are known flaky tests we haven't been able to figure out, so don't worry about those :)