Add external/musl to the bp2build allowlists

Add external/musl to the bp2build allowlists.  Disable libc_musl_sysroot,
which fails due to a $(locations) reference to a glob that is
unsupported in bazel.

Bug: 259266326
Test: //external/musl:libc_musl builds
Change-Id: Icf0ea2f89854ba3605096dcaa95c1c829af34dc3