Switch the order in which the version-specific attributes are combined
with args. Otherwise, if args such as 'branch' are overridden by an
overlay or other derivation, the new value is replaced with the
original when the sets are combined with //.
This commit fixes missing fc-cache binary from make-fonts-cache.nix build:
```
builder for '/nix/store/az48nr8gdqrw3fliddmi82ghj2ljxrj4-fc-cache.drv' failed with exit code 127; last 1 log lines:
/nix/store/p3z1lgsi7xymvl7akg531ikwiisqs4x5-stdenv-linux/setup: line 1299: fc-cache: command not found
cannot build derivation '/nix/store/swaxvjsf8h0rsmm9kigp6j3f5q5h4nvg-fc-00-nixos-cache.conf.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/wiaiv0pq7w1xm2i2fqp2ngd1ljb4n6n9-fontconfig-conf.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/4zhiwpiyccs0rs26bs3q0w8fwaxrrgw0-fontconfig-etc.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/xhvljdp9b00fbkapx6cbfs4sjdh49qwv-etc.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/w63q0n0vh7vkdfrjmhb41qy1alx7qa8s-nixos-system-nixos-19.09.git.c814289.drv': 1 dependencies couldn't be built
```
This is required for programs using rocksdb and and typeinfo.
Otherwise, linking them fails with errors like this (that's ceph):
/nix/store/cg0k49h66nkdqx6ccwnqr0i4q0fnfznc-binutils-2.31.1/bin/ld: ../../lib/libos.a(RocksDBStore.cc.o):(.data.rel.ro._ZTIN12RocksDBStore14RocksWBHandlerE[_ZTIN12RocksDBStore14RocksWBHandlerE]+0x10): undefined reference to `typeinfo for rocksdb::WriteBatch::Handler'
/nix/store/cg0k49h66nkdqx6ccwnqr0i4q0fnfznc-binutils-2.31.1/bin/ld: ../../lib/libos.a(RocksDBStore.cc.o):(.data.rel.ro._ZTIN12RocksDBStore19MergeOperatorRouterE[_ZTIN12RocksDBStore19MergeOperatorRouterE]+0x10): undefined reference to `typeinfo for rocksdb::AssociativeMergeOperator'
/nix/store/cg0k49h66nkdqx6ccwnqr0i4q0fnfznc-binutils-2.31.1/bin/ld: ../../lib/libos.a(RocksDBStore.cc.o):(.data.rel.ro._ZTIN12RocksDBStore19MergeOperatorLinkerE[_ZTIN12RocksDBStore19MergeOperatorLinkerE]+0x10): undefined reference to `typeinfo for rocksdb::AssociativeMergeOperator'
/nix/store/cg0k49h66nkdqx6ccwnqr0i4q0fnfznc-binutils-2.31.1/bin/ld: ../../lib/libos.a(RocksDBStore.cc.o):(.data.rel.ro._ZTI17CephRocksdbLogger[_ZTI17CephRocksdbLogger]+0x10): undefined reference to `typeinfo for rocksdb::Logger'
/nix/store/cg0k49h66nkdqx6ccwnqr0i4q0fnfznc-binutils-2.31.1/bin/ld: ../../lib/libos.a(BlueRocksEnv.cc.o):(.data.rel.ro._ZTI12BlueRocksEnv[_ZTI12BlueRocksEnv]+0x10): undefined reference to `typeinfo for rocksdb::EnvWrapper'
/nix/store/cg0k49h66nkdqx6ccwnqr0i4q0fnfznc-binutils-2.31.1/bin/ld: ../../lib/libos.a(BlueRocksEnv.cc.o):(.data.rel.ro._ZTI23BlueRocksSequentialFile[_ZTI23BlueRocksSequentialFile]+0x10): undefined reference to `typeinfo for rocksdb::SequentialFile'
/nix/store/cg0k49h66nkdqx6ccwnqr0i4q0fnfznc-binutils-2.31.1/bin/ld: ../../lib/libos.a(BlueRocksEnv.cc.o):(.data.rel.ro._ZTI25BlueRocksRandomAccessFile[_ZTI25BlueRocksRandomAccessFile]+0x10): undefined reference to `typeinfo for rocksdb::RandomAccessFile'
/nix/store/cg0k49h66nkdqx6ccwnqr0i4q0fnfznc-binutils-2.31.1/bin/ld: ../../lib/libos.a(BlueRocksEnv.cc.o):(.data.rel.ro._ZTI21BlueRocksWritableFile[_ZTI21BlueRocksWritableFile]+0x10): undefined reference to `typeinfo for rocksdb::WritableFile'
/nix/store/cg0k49h66nkdqx6ccwnqr0i4q0fnfznc-binutils-2.31.1/bin/ld: ../../lib/libos.a(BlueRocksEnv.cc.o):(.data.rel.ro._ZTI17BlueRocksFileLock[_ZTI17BlueRocksFileLock]+0x10): undefined reference to `typeinfo for rocksdb::FileLock'
I think those deps could be made to build, but I didn't want to get
bogged down investigating further. "Use flags" are always a good thing,
so this is fine for now.
