Reinstate the checkout

[bitdivine]

Motivation

When cleaning up the commit comparison tool I managed to delete the line where the local code is checked out.

Changes

• Check out the commit being tested before running the local docker build.

Tests

Running on a range of commits shows the repo being checked out:

$ ./scripts/nns-dapp/frontend-reproducibility-report
Previous HEAD position was 027bd72e0 E2E: Stake an SNS neuron (#2316)
HEAD is now at c01f19136 Better command line url test (#2324)
Previous HEAD position was c01f19136 Better command line url test (#2324)
HEAD is now at 38e3a55e0 fix: broken gap by tid (#2337)
Previous HEAD position was 38e3a55e0 fix: broken gap by tid (#2337)
HEAD is now at 388abdde1 Log pre-upgrade cycle count (#2335)
Previous HEAD position was 388abdde1 Log pre-upgrade cycle count (#2335)
HEAD is now at 2de7376d9 Style improvement for release candidate (#2334)
Previous HEAD position was 2de7376d9 Style improvement for release candidate (#2334)
HEAD is now at 51e4e1f48 Potential work-around for Rollup cyclic dependency bug (#2332)