Android Editor builds fail due to storage problems

**Bug description**

As you can see in the GitHub Actions, the last few 6.3.X builds failed to complete.
https://github.com/game-ci/docker/actions/runs/21362813556

It looks like the root cause is that the build process runs out of diskspace during a caching process.

**Additional details**

I am not very familiar with this workflow and I did ask ChatGPT for help, so I am not sure if this is a good suggestion. That's why I didn't make a PR but an issue instead.
AI suggests to offload buildx caching from tmp/ to mnt/ which might gives us more diskspace.

```
diff --git a/.github/workflows/new-ubuntu-post-2019-2-editor-image-requested.yml b/.github/workflows/new-ubuntu-post-2019-2-editor-image-requested.yml
index 1234567..abcdef0 100644
--- a/.github/workflows/new-ubuntu-post-2019-2-editor-image-requested.yml
+++ b/.github/workflows/new-ubuntu-post-2019-2-editor-image-requested.yml
@@ -95,6 +95,13 @@ jobs:
- name: Free disk space
run: .github/workflows/scripts/free_disk_space.sh

+ ############################
+ # Prepare buildx cache #
+ ############################
+ - name: Prepare buildx cache dir
+ run: |
+ sudo mkdir -p /mnt/buildx-cache
+ sudo chown -R $USER:$USER /mnt/buildx-cache
+
############################
# Pull previous images #
############################
@@ -118,8 +125,8 @@ jobs:
changeSet=${{ github.event.client_payload.changeSet }}
module=${{ matrix.targetPlatform }}
- cache-from: type=local,src=/tmp/.buildx-cache
- cache-to: type=local,dest=/tmp/.buildx-cache
+ cache-from: type=local,src=/mnt/buildx-cache
+ cache-to: type=local,dest=/mnt/buildx-cache
push: true
tags: |
unityci/editor:${{ github.event.client_payload.editorVersion }}-${{ matrix.targetPlatform }}-${{ github.event.client_payload.repoVersionFull }}
@@ -152,8 +159,8 @@ jobs:
changeSet=${{ github.event.client_payload.changeSet }}
module=${{ matrix.targetPlatform }}
- cache-from: type=local,src=/tmp/.buildx-cache
- cache-to: type=local,dest=/tmp/.buildx-cache
+ cache-from: type=local,src=/mnt/buildx-cache
+ cache-to: type=local,dest=/mnt/buildx-cache
push: true
tags: |
unityci/editor:${{ github.event.client_payload.editorVersion }}-${{ matrix.targetPlatform }}-${{ github.event.client_payload.repoVersionFull }}
```