fix(3p/nix): allocate the vSourceExpr via uncollectable memory r/1352

Previously the memory would occasionally be collected during eval since
the GC doesn't consider the member variable as alive / doesn't scan the
region of memory where the pointer lives.

By using the traceable_allocator<T> allocator provided by Boehm GC we
can ensure the memory isn't collected. It should be properly freed when
SourceExprCommand goes out of scope.

Backported from:
https://github.com/NixOS/nix/commit/d2c371927e45c1d4e4f7e4ad331a3f68d67d5469

Change-Id: I1f7c745dbc66c7164bee50f4d9b0d437dbc7dd51
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1251
Tested-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>

0 files changed, 0 insertions, 0 deletions