Update devbox package install script (#6)

### Summary
devbox CLI has deprecated support for 
`devbox shell -- <cmd>`

It is replaced by `devbox run -- <cmd>`.

In the future when `devbox install` command is available, we can switch
to using that instead.

Related issue: jetify-com/devbox#790

### How was it tested?
CICD

Author: LucilleH

.github/workflows/test.yaml

@@ -35,7 +35,7 @@ jobs:
       - name: Install devbox
         uses: ./
         with:
-          devbox-version: '0.2.2'
+          devbox-version: 0.4.4
           project-path: 'testdata'
 
   test-action-with-cache:

action.yml

@@ -66,7 +66,7 @@ runs:
       if: steps.cache-devbox-cli.outputs.cache-hit != 'true'
       shell: bash
       run: |
-        DEVBOX_USE_VERSION="${{ env.latest_version }}"
+        export DEVBOX_USE_VERSION="${{ env.latest_version }}"
         curl -fsSL https://get.jetpack.io/devbox | FORCE=1 bash
 
         version=$(devbox version)
@@ -81,4 +81,4 @@ runs:
       run: |
         NIX_INSTALLER_NO_CHANNEL_ADD=1
         NIX_BUILD_SHELL=/bin/bash
-        devbox shell --config=${{ inputs.project-path }} -- echo "Packages installed!"
+        devbox run --config=${{ inputs.project-path }} -- echo "Packages installed!"