[tmpnet] Add --start-network to support hypersdk MODE=run

[marun]

Why this should be merged

Hypersdk tooling has long supported supplying MODE=run to its run.sh script to start a network and leave it running without executing any tests. When hypersdk switched to tmpnet, the run script was updated to specify --reuse-network and run a trivial test, but the use of --reuse-network risks confusion if a user expects the resulting network to reflect the latest binary state. Adding explicit support for always starting a new network avoids this risk.

How this works

• Add new flag --start-network to the tmpnet env flags
• If the flag is provided, start a new network, avoid stopping it on cleanup, and exist before running any tests

How this was tested

CI for regression, locally to check that a new network was started and stayed running

[aaronbuchwald]

LGTM