Add missing shebang to "securedrop.Proxy"

[deeplow]

This misisng shebang lead to an execve error in Qubes 4.3

Fixes #2770

Test plan

In 4.2:

• try-client-pr 1461
• ensure client logs in (otherwise it would fail the login)

In 4.3: WIP

Checklist

This change accounts for:

• testing changes on Qubes as needed (especially changes related to cryptography, export, disposable VM use, or complex UI changes)
• any needed updates to the AppArmor profile for files beyond the application code
• any needed self-contained database migrations (including testing against a clean test database from main)

[deeplow]

Marking as ready for review, but under the understanding that to verify in 4.3 without doing a 4.3 install is not yet ready.

[legoktm]

Does securedrop.Log need the same fix?

[deeplow]

Good point.

[legoktm]

Verified that I could log in and that logs were flowing.