git2-rs unable to find libgit2 built with `-DEXPERIMENTAL_SHA256=ON`

I tried to building gram using export LIBGIT2_NO_VENDOR=1 and with a build of libgit2 with the following flag -DEXPERIMENTAL_SHA256=ON. I then came across this error

|B│      Running `/mason/build/x86_64/2.2.0.tar.gz/target/release/build/libgit2-sys-1f279ea7f9a9f4f3/build-script-build`
│B│ warning: libgit2-sys@0.18.4+1.9.3: no GIT_EXPERIMENTAL_SHA256 constant in headers
│B│ error: failed to run custom build command for `libgit2-sys v0.18.4+1.9.3`
│B│
│B│ Caused by:
│B│   process didn't exit successfully: `/mason/build/x86_64/2.2.0.tar.gz/target/release/build/libgit2-sys-1f279ea7f9a9f4f3/build-script-build` (exit status: 101)
│B│   --- stdout
│B│   cargo:rustc-check-cfg=cfg(libgit2_vendored,libgit2_experimental_sha256,)
│B│   cargo:rerun-if-env-changed=LIBGIT2_NO_VENDOR
│B│   cargo:rerun-if-env-changed=LIBGIT2_EXPERIMENTAL_NO_PKG_CONFIG
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_x86_64-unknown-linux-gnu
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_x86_64_unknown_linux_gnu
│B│   cargo:rerun-if-env-changed=HOST_PKG_CONFIG
│B│   cargo:rerun-if-env-changed=PKG_CONFIG
│B│   cargo:rerun-if-env-changed=LIBGIT2_EXPERIMENTAL_STATIC
│B│   cargo:rerun-if-env-changed=LIBGIT2_EXPERIMENTAL_DYNAMIC
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_ALL_STATIC
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_ALL_DYNAMIC
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_PATH_x86_64-unknown-linux-gnu
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_PATH_x86_64_unknown_linux_gnu
│B│   cargo:rerun-if-env-changed=HOST_PKG_CONFIG_PATH
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_PATH
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_LIBDIR_x86_64-unknown-linux-gnu
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_LIBDIR_x86_64_unknown_linux_gnu
│B│   cargo:rerun-if-env-changed=HOST_PKG_CONFIG_LIBDIR
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_LIBDIR
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR_x86_64-unknown-linux-gnu
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR_x86_64_unknown_linux_gnu
│B│   cargo:rerun-if-env-changed=HOST_PKG_CONFIG_SYSROOT_DIR
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR
│B│   cargo:rerun-if-env-changed=SYSROOT
│B│   cargo:rerun-if-env-changed=LIBGIT2_EXPERIMENTAL_STATIC
│B│   cargo:rerun-if-env-changed=LIBGIT2_EXPERIMENTAL_DYNAMIC
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_ALL_STATIC
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_ALL_DYNAMIC
│B│   cargo:rustc-link-search=native=/usr/lib
│B│   cargo:rustc-link-lib=git2-experimental
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_x86_64-unknown-linux-gnu
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_x86_64_unknown_linux_gnu
│B│   cargo:rerun-if-env-changed=HOST_PKG_CONFIG
│B│   cargo:rerun-if-env-changed=PKG_CONFIG
│B│   cargo:rerun-if-env-changed=LIBGIT2_EXPERIMENTAL_STATIC
│B│   cargo:rerun-if-env-changed=LIBGIT2_EXPERIMENTAL_DYNAMIC
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_ALL_STATIC
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_ALL_DYNAMIC
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_PATH_x86_64-unknown-linux-gnu
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_PATH_x86_64_unknown_linux_gnu
│B│   cargo:rerun-if-env-changed=HOST_PKG_CONFIG_PATH
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_PATH
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_LIBDIR_x86_64-unknown-linux-gnu
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_LIBDIR_x86_64_unknown_linux_gnu
│B│   cargo:rerun-if-env-changed=HOST_PKG_CONFIG_LIBDIR
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_LIBDIR
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR_x86_64-unknown-linux-gnu
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR_x86_64_unknown_linux_gnu
│B│   cargo:rerun-if-env-changed=HOST_PKG_CONFIG_SYSROOT_DIR
│B│   cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR
│B│   cargo:warning=no GIT_EXPERIMENTAL_SHA256 constant in headers
│B│
│B│   --- stderr
│B│
│B│   thread 'main' (12205) panicked at /mason/build/x86_64/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/libgit2-sys-0.18.4+1.9.3/build.rs:100:13:
│B│   The environment variable `LIBGIT2_NO_VENDOR` has been set but no compatible system libgit2 could be found.
│B│   The build is now aborting. To disable, unset the variable or use `LIBGIT2_NO_VENDOR=0`.
│B│
│B│   stack backtrace:
│B│      0: __rustc::rust_begin_unwind
│B│      1: core::panicking::panic_fmt
│B│      2: build_script_build::main
│B│      3: core::ops::function::FnOnce::call_once
│B│   note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
│B│ warning: build failed, waiting for other jobs to finish...

I dont believe gram (or git2-rs?) was able to find my build of libgit2 using the experimental option, what do? Also is the issue applicable here or in libgit2?

Start by reading the libgit2-sys build.rs entry point and trace how LIBGIT2_NO_VENDOR, pkg-config, and the experimental SHA-256 header check identify the system library. Reproduce the build with the reported environment and inspect the selected libgit2 headers and pkg-config metadata; done means the compatible experimental library is detected and the build proceeds without the panic.