fixes #72

mere dev build (and unprivileged mere install) aborted when installing a
dependency from a remote repo: after the package was successfully built,
verified, and admitted to the store, the install tried to write rollback
state into /mere/cache/repos/<...>/rollback_state.kdl — a root-owned path
— and failed with a FileSystem error ("failed to update rollback state").

scanElfMetadata read the .dynamic section into a byte buffer via
allocator.alloc (1-byte aligned), then @alignCast'd each entry to
*Elf64_Dyn. @alignCast asserts the type's 8-byte alignment and panics
in safe builds when the buffer base isn't 8-byte aligned — which it
isn't guaranteed to be. This crashed `mere dev build` during package
artifact scanning (hit while packaging pixman).
Read d_tag and d_val by offset with std.mem.readInt instead, matching
how the rest of elf.zig parses ELF structures. No alignment assumption,
and endianness is explicit (native_endian, already guaranteed by the
scan's earlier endian guard).