fix: Add build tags to loadPackage

[kevjumba]

If buildFirst flag is set when loading packages, makes sure that the build flags are imported correctly to prevent intermediate errors like these:

Issue is resolved later when package is actually built but we're missing build tags beforehand.

[kevjumba]

Quick fix: PTAL @rcoreilly Thanks!

[kevjumba]

@rcoreilly Please take a look at this if you have time!

[rcoreilly]

all looks good and apologies for the long delay!