druntime tests can fail due to not having rebuilt phobos

[dhasenan]

https://buildkite.com/dlang/dmd/builds/3182#c8f1ded2-a08a-4459-b71a-6dc737caae38 as an example. The error listed is:

benchmark/runbench.d:(.text._D3std3uni__T14findUnicodeSetS_DQBe8internal14unicode_tables6blocks3tabFNaNdNfZAySQDcQByQBs15UnicodePropertyTaZQDuFNaNfMxAaZl[_D3std3uni__T14findUnicodeSetS_DQBe8internal14unicode_tables6blocks3tabFNaNdNfZAySQDcQByQBs15UnicodePropertyTaZQDuFNaNfMxAaZl]+0x11): undefined reference to `_D3std8internal14unicode_tables6blocks3tabFNaNdNfZAySQBzQByQBs15UnicodeProperty'

This is because the PR under test changed attribute propagation, which changed the mangled name in std.internal.unicode_tables. The new mangled name didn't exist in the pre-existing libphobos, so there was a linker error when trying to build the benchmark tool.

The druntime build should build phobos and use that newly built phobos to build the benchmark tool.