Attempt 1:
these derivations will be built:
  /nix/store/p9ppvqkpa4wn4v05p9b1w8sxr6545zym-grin-0.1.drv
building '/nix/store/p9ppvqkpa4wn4v05p9b1w8sxr6545zym-grin-0.1.drv'...
unpacking sources
unpacking source archive /nix/store/awafsmp7vywnfrh16qafsn186x2q4mh7-grin
source root is grin
unpacking source archive /nix/store/wiyq1x1r8pvvy5grhp060c0bnwqm74d1-grin-0.1-vendor
patching sources
configuring
building
Running cargo build --release
error: failed to load source for a dependency on `secp256k1zkp`

Caused by:
  Unable to update https://github.com/mimblewimble/rust-secp256k1-zkp?tag=grin_integration_15#eff11ead

Caused by:
  failed to open: /homeless-shelter/.cargo/git/.cargo-lock-git

Caused by:
  Permission denied (os error 13)
builder for '/nix/store/p9ppvqkpa4wn4v05p9b1w8sxr6545zym-grin-0.1.drv' failed with exit code 101
error: build of '/nix/store/p9ppvqkpa4wn4v05p9b1w8sxr6545zym-grin-0.1.drv'

Attempt 2 after adding "export HOME=$PWD" in postUnpack:

these derivations will be built:
  /nix/store/aaz8aksqwkxx2d4y5ns78kv9c8vr9l05-grin-0.1.drv
building '/nix/store/aaz8aksqwkxx2d4y5ns78kv9c8vr9l05-grin-0.1.drv'...
unpacking sources
unpacking source archive /nix/store/awafsmp7vywnfrh16qafsn186x2q4mh7-grin
source root is grin
unpacking source archive /nix/store/wiyq1x1r8pvvy5grhp060c0bnwqm74d1-grin-0.1-vendor
patching sources
configuring
building
Running cargo build --release
    Updating git repository `https://github.com/mimblewimble/rust-secp256k1-zkp`
error: failed to load source for a dependency on `secp256k1zkp`

Caused by:
  Unable to update https://github.com/mimblewimble/rust-secp256k1-zkp?tag=grin_integration_15#eff11ead

Caused by:
  failed to clone into: /build/.cargo/git/db/rust-secp256k1-zkp-2d57249f422a426c

Caused by:
  attempting to update a git repository, but --frozen was specified
builder for '/nix/store/aaz8aksqwkxx2d4y5ns78kv9c8vr9l05-grin-0.1.drv' failed with exit code 101
error: build of '/nix/store/aaz8aksqwkxx2d4y5ns78kv9c8vr9l05-grin-0.1.drv' failed