Softfail by default

Use softfail setting for importing remote packages.  This resolves issues such as

fatal error: Can't find overlay named NaOH.link in [:]: file atpkg/src/Package.swift, line 375

(when it is not required as it's a test-only dependency)
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passed #6892
autoscale-linux
linux

00:00:26

passed #6893
openswift atbuild
osx

00:00:12