touchscreen: fix env var name (#3788)

oh right, `NEXT_PUBLIC_` prefix, doh

### Change Type

<!-- ❗ Please select a 'Scope' label ❗️ -->

- [x] `sdk` — Changes the tldraw SDK
- [ ] `dotcom` — Changes the tldraw.com web app
- [ ] `docs` — Changes to the documentation, examples, or templates.
- [ ] `vs code` — Changes to the vscode plugin
- [ ] `internal` — Does not affect user-facing stuff

<!-- ❗ Please select a 'Type' label ❗️ -->

- [x] `bugfix` — Bug fix
- [ ] `feature` — New feature
- [ ] `improvement` — Improving existing features
- [ ] `chore` — Updating dependencies, other boring stuff
- [ ] `galaxy brain` — Architectural changes
- [ ] `tests` — Changes to any test code
- [ ] `tools` — Changes to infrastructure, CI, internal scripts,
debugging tools, etc.
- [ ] `dunno` — I don't know

apps/dotcom/src/pages/public-touchscreen-side-panel.tsx

@@ -11,7 +11,7 @@ export function Component() {
 			}
 
 			const session = await (window as any).meet.addon.createAddonSession({
-				cloudProjectNumber: `${process.env.GOOGLE_CLOUD_PROJECT_NUMBER}`,
+				cloudProjectNumber: `${process.env.NEXT_PUBLIC_GOOGLE_CLOUD_PROJECT_NUMBER}`,
 			})
 			const sidePanelClient = await session.createSidePanelClient()
 			await sidePanelClient.setCollaborationStartingState({