tvix: Double free when using tvix as a remote builder

to reproduce:

• make tvix the nix.package on a machine
• set that machine as a remote builder
• attempt to build a derivation

1. we thought https://cl.tvl.fyi/c/depot/+/2104 would fix this, but it didn't

   grfn at 2020-11-18T13·39+00

2. https://cl.tvl.fyi/c/depot/+/2106 fixes this

   grfn at 2020-11-18T14·44+00

3. grfn closed this issue at 2020-11-20T19·39+00