Fix FORCE_INSTALL when using simple install commands (#1865)

Resolves #1864 

This change updates `get-install.sh` to preserve the `FORCE_INSTALL`
environment variable when running `sudo ./install`.

This allows users to set `FORCE_INSTALL` before running the simple
install commands.

To test live on a device, install Raspberry Pi OS Bullseye 32-bit and
run:

```bash
export FORCE_INSTALL=1
```

And then:

```bash
curl \
  --silent \
  --show-error \
  https://raw.githubusercontent.com/tiny-pilot/tinypilot/refs/heads/1864-force_install-doesnt-work-as-expected/get-tinypilot.sh | \
    bash - && \
  sudo reboot
```

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get-tinypilot.sh

@@ -160,7 +160,7 @@ fi
 # Run install.
 pushd "${INSTALLER_DIR}"
 sudo \
-  TMPDIR="${TMPDIR}" \
+  --preserve-env=TMPDIR,FORCE_INSTALL \
   ./install
 
 } # Prevent the script from executing until the client downloads the full file.