Skip building build tasks if they already exist

[papeh]

Build tasks rarely change, so we can usually skip building them.

Also, building build tasks doesn't work when the DLL is locked, as has often been the case on my machine this week.

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This change is 

[github-actions]

NUnit Tests

    1 files  ±0      1 suites  ±0   6m 23s ⏱️ +13s
4 103 tests ±0  4 032 ✅ ±0  71 💤 ±0  0 ❌ ±0 
4 112 runs  ±0  4 041 ✅ ±0  71 💤 ±0  0 ❌ ±0 

Results for commit 89d512d. ± Comparison against base commit 1eacfff.