Small fixes for recent work in VSIX up-to-date checks and path detection

[sharwell]

• [x] Only hook up VSIX up-to-date check when VSToolsPath is defined
• [x] Ensure globalPackagesFolder ends with a directory separator
• [x] Update Sign.proj to use a NuGet-defined package cache location instead of assuming a prior step placed it at a hard-coded location

[premun]

@sharwell a quick note - we squash merge in Arcade instead of merging only