interp: avoid more unnecessary string copies
interp: avoid more unnecessary string copies

name   old time/op    new time/op    delta
Run-4    1.37ms ± 1%    1.37ms ± 1%    ~     (p=0.429 n=5+6)

name   old alloc/op   new alloc/op   delta
Run-4     163kB ± 0%     162kB ± 0%  -0.09%  (p=0.002 n=6+6)

name   old allocs/op  new allocs/op  delta
Run-4       622 ± 0%       586 ± 0%  -5.79%  (p=0.002 n=6+6)

mvdan/sh,mvdan/sh,mvdan/sh,mvdan/sh