Fix PortableBuild comparisons

[jkoritzinsky]

Compare PortableBuild against false instead of != true as a build is by default portable (and we don't pass PortableBuild=true in a number of scenarios).

Also fix a typo in the source-build step script.

Contributes to unblocking dotnet/sdk#45042

To double check:

• The right tests are in and the right validation has happened. Guidance: https://github.com/dotnet/arcade/blob/main/Documentation/Validation.md

[Copilot]

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Files not reviewed (1)

• src/Microsoft.DotNet.Build.Tasks.Installers/build/installer.targets: Language not supported