[Impeller] Can't launch impeller_unittests with RenderDoc on Windows.

I tried invoking RenderDoc using both the GUI and CLI. For GUI, process launches and immediately closes with no output. For CLI the same thing happens and RenderDoc just outputs `Launched as ID 0`:
```
/c/Program\ Files/RenderDoc/renderdoccmd.exe capture "C:\Users\bdero\projects\flutter\engine\src\out\host_debug_unopt
\impeller_unittests.exe" "--gtest_filter="Play/AiksTest.CanRenderLinearGradientRepeat/Vulkan" --enable_playground"
Launching 'C:\Users\bdero\projects\flutter\engine\src\out\host_debug_unopt\impeller_unittests.exe' with params: "--gtest_filter=Play/AiksTest.CanRenderLinearGradientRepeat/Vulkan --enable_playground"
Launched as ID 0
```

Just trying to repro the crash that @chinmaygarde was running into on Windows. Haven't done any further investigation.