Allow Bypass list to be passed into the proxy

[ZEisinger]

Required for actions/setup-dotnet#71. Agents can set proxy settings for their runs which include addresses which should not go through the proxy. This script doesn't allow that value to be specified for windows. In the shell script this is handled by wget and curl both honoring the environment variables https_proxy and no_proxy.

[bekir-ozturk]

Hi @ZEisinger ,
Thank you very much for the PR.
Can you give me a bit more info on your use cases? For instance, how do you acquire the script? Do you have a local copy that you retain or are you downloading it from www.dot.net/v1/dotnet-install.ps1 before each run? Are you blocked by this PR?
Unfortunately, we currently don't have a strong testing mechanism to ensure that the changes to the scripts won't break any existing scenarios. Therefore, we refrain from making changes.

[ZEisinger]

@bozturkMSFT Actions follow a consistent pattern for declaring and consuming proxy settings which includes the ability to specify when to skip the proxy (no_proxy environment variable or BypassList in dotnet). The windows install script has the ability to specify the proxy address to use, but there no way of specifying when to skip the proxy.

The action currently consumes these scripts from a local copy, but checks the feed for updates in CI tests. I know there is typically some lead time on changes to these script so I opened the PR here before my PR is ready in setup-dotnet. If needed, I can disable the CI test temporarily until this change is available at www.dot.net.