fix(tvix): Use mutable field for assigning proto fields r/1885

This was accidentally using the proto arena API to assign the derivation
field of a BuildDerivationRequest. We *thought* this was causing a
double free, but even with this change that's still happening. That
said, this change is probably still a good idea since it's using the
proto API as intended.

References: b/64
Change-Id: I950a4eafb214e9113639ea54d2dfd4659b7be931
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2104
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

0 files changed, 0 insertions, 0 deletions