dotnet-core-sdk: Fix environment setup

[RemiLacroix-IDRIS]

The "DOTNET_CLI_TELEMETRY_OPTOUT" environment variable should be defined when using the product, not when installing it (the installation phase is just extract the files anyway).

Also use ~ instead of - to check for the variant and fix the second argument for env.set which should also be a string.

[spackbot-app]

Hi @RemiLacroix-IDRIS! I noticed that the following package(s) don't yet have maintainers:

• dotnet-core-sdk

Are you interested in adopting any of these package(s)? If so, simply add the following to the package class:

    maintainers("RemiLacroix-IDRIS")

If not, could you contact the developers of this package and see if they are interested? You can quickly see who has worked on a package with spack blame:

$ spack blame dotnet-core-sdk

Thank you for your help! Please don't add maintainers without their consent.

You don't have to be a Spack expert or package developer in order to be a "maintainer," it just gives us a list of users willing to review PRs or debug issues relating to this package. A package can have multiple maintainers; just add a list of GitHub handles of anyone who wants to volunteer.

[tldahlgren]

LGTM