Make IIS benchmarks work again

[jkotalik]

Depends on dotnet/aspnetcore#14112 (I may have to change this PR for only 5.0 as it's going in master first).

[jkotalik]

@sebastienros this works. I'm hard coding this version until it works in 3.0.

[jkotalik]

Current numbers:

RequestsPerSecond:           187,027
Max CPU (%):                 89
WorkingSet (MB):             169
Avg. Latency (ms):           11.4
Startup (ms):                514
First Request (ms):          370.28
Latency (ms):                0.5
Total Requests:              2,824,049
Duration: (ms)               15,100
Socket Errors:               0
Bad Responses:               0
Build Time (ms):             7,121
Published Size (KB):         105,314
SDK:                         3.0.100-rc2-014256
Runtime:                     5.0.0-alpha1.19465.2
ASP.NET Core:                3.0.0-rc2.19461.12