approach env: clean up InstinctAct

[garymm]

• Don't rely on double consume to set ShouldGate. What really matters is that the agent currently has a goal engaged (hasGated) and just got a US (ev.US != noUS).
• Only set ShouldGate once. It's easier to understand if it doesn't change inside the function body IMHO.

Change-Id: I0334ec86acbbeafcf36a5b73824d80d396cf625b

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.02 :warning:

Comparison is base (0acf0aa) 30.62% compared to head (69d331d) 30.60%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## Ibb1c6d5d5d79f8c6002931dde602df9786777ad6 #260 +/- ## ============================================================================= - Coverage 30.62% 30.60% -0.02% ============================================================================= Files 79 79 Lines 15152 15149 -3 ============================================================================= - Hits 4640 4637 -3 Misses 10255 10255 Partials 257 257 ``` | [Impacted Files](https://app.codecov.io/gh/emer/axon/pull/260?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=emer) | Coverage Δ | | |---|---|---| | [examples/boa/approach\_env.go](https://app.codecov.io/gh/emer/axon/pull/260?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=emer#diff-ZXhhbXBsZXMvYm9hL2FwcHJvYWNoX2Vudi5nbw==) | `92.10% <100.00%> (-0.11%)` | :arrow_down: |

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