[repo-assist] Fall back to User/Machine scoped environment variables in paket.dependencies

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Root cause

EnvironmentVariable.Create (used to resolve %VAR% references in paket.dependencies, e.g. for credentials) called Environment.GetEnvironmentVariable(name), which only reads the current process's environment block. Variables set at the User or Machine scope via [System.Environment]::SetEnvironmentVariable(name, value, 'User') are persisted (e.g. in the Windows registry) but are not reflected in an already-running process's environment — a new process must be started to see them. This caused Paket to warn environment variable '%VAR%' not found even though the variable was correctly set, as reported in #3562.

Fix

EnvironmentVariable.Create now falls back to explicitly querying the User and Machine EnvironmentVariableTarget scopes when the Process scope lookup misses, before giving up and warning. PlatformNotSupportedException (thrown on non-Windows platforms, where these scopes don't exist) is caught so the fallback is a safe no-op there.

Trade-offs

• Slightly more work per variable lookup (up to 2 extra GetEnvironmentVariable calls) only in the miss case; negligible cost since credential resolution happens rarely (parse-time).
• No change in behavior for variables already resolvable at Process scope — same value, same order of preference (Process wins if a variable is defined at multiple scopes, matching normal Windows semantics).

Closes #3562

Test Status

• Added two unit tests in PackageSourceSpecs.fs — resolving a process-level env var, and warning/empty-value for an unknown var.
• dotnet test tests/Paket.Tests/Paket.Tests.fsproj --filter "FullyQualifiedName~PackageSourceSpecs": 16/16 passed on net9.0. The net461 target could not execute in this sandbox (mono host not installed) — this is an environment limitation, not a code failure; the same assembly built successfully for both target frameworks.

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