./boot flakiness in CI due to "Error: libraries/deepseq/LICENSE doesn't exist"

In a recent pipeline 3 of the 4 build stage jobs failed due to this issue:

• hadrian-ghc-in-ghci
• release-x86_64-linux-deb9-dwarf
• validate-x86_64-linux-deb9-debug

Each time the error looks like this:

$ ./boot
Error: libraries/deepseq/LICENSE doesn't exist
Maybe you haven't run 'git submodule update --init'?

Up to this point the logs seem entirely unremarkable to me.

I've been seeing this failure for a few weeks already.