Use Ubuntu 18.04 and dotnet-buildtools-prereq docker images for enterprise linux pipeline

[akoeplinger]

Ubuntu 16.04 is no longer available on Azure Pipelines, move to 18.04 1ES pool and the Docker images from dotnet-buildtools-prereq.

Also add a workaround for #34649 which we hit during the build.

Replaces #60261

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Issue Details

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Ubuntu 16.04 is no longer available on Azure Pipelines, move to 18.04 and the Docker images from dotnet-buildtools-prereq.

Author:	akoeplinger
Assignees:	-
Labels:	area-Infrastructure-libraries
Milestone:	-

[akoeplinger]

This replaces #60261 /cc @jkoritzinsky

I hit the same build error as that one and found out that something is blocking resolving dotnet.microsoft.com so the dotnet-install.sh can't be fetched in the docker container during the build.

It doesn't happen locally and after a bit of searching it seems specific to the Azure Pipelines hosted pool environment (actions/runner-images#798) and that one is believed to be due to a firewall rule blocking the traffic (Azure/WALinuxAgent#1673).

The other pipeline building inside docker is stress-http and that one was moved to the non-hosted pool a while ago in #35011 so this makes us consistent with that pipeline.

To get the build to work after that I also had to add a workaround for #34649 which we hit during the build.

/cc @dotnet/ncl

[safern]

LGTM

[jkoritzinsky]

Can you queue a run of the pipeline on this PR? I don’t see it in the checks.

[akoeplinger]

@jkoritzinsky it was triggered and is green, you must've missed it (it's called runtime-libraries enterprise-linux in the checks). See https://dev.azure.com/dnceng/public/_build/results?buildId=1446008&view=results

[akoeplinger]

/backport to release/6.0

[akoeplinger]

/backport to release/6.0

[github-actions]

Started backporting to release/6.0: https://github.com/dotnet/runtime/actions/runs/1602706792