Since this feature's inception, we've compiled a binary that uses OpenSSL
to perform PBKDF-SHA512 during the extra-utils build. In addition to this
being inefficient, it broke as of 6ea1a2a1be,
which switched the extra-utils derivation to use stdenvNoCC.
For now, I think the path of least resistence is to move the pbkdf-sha512
build to its own derivation, to fix the breakage, as well as improving
the efficiency of the extra-utils build.
(I do believe that at some point, we should revisit this binary -- perhaps
rewriting it -- as Clang even just on its default settings emits more
warnings than you'd want to see in a security-related codebase when
compiling it.)