Fix docsrs build

[dsykes16]

Fix docs.rs build by removing exclusion on protobuf code generation when DOCS_RS environment variable is set.

This issue appears to have been introduced in #138.

Docs.rs build log:

# rustc version
rustc 1.90.0-nightly (ba7e63b63 2025-07-29)# docs.rs version
docsrs 0.6.0 (e18d034e 2025-07-29)# build log
[INFO] running `Command { std: "docker" "create" "-v" "/home/cratesfyi/workspace/builds/spiffe-0.6.7/target:/opt/rustwide/target:rw,Z" "-v" "/home/cratesfyi/workspace/builds/spiffe-0.6.7/source:/opt/rustwide/workdir:ro,Z" "-v" "/home/cratesfyi/workspace/cargo-home:/opt/rustwide/cargo-home:ro,Z" "-v" "/home/cratesfyi/workspace/rustup-home:/opt/rustwide/rustup-home:ro,Z" "-e" "SOURCE_DIR=/opt/rustwide/workdir" "-e" "CARGO_TARGET_DIR=/opt/rustwide/target" "-e" "DOCS_RS=1" "-e" "CARGO_HOME=/opt/rustwide/cargo-home" "-e" "RUSTUP_HOME=/opt/rustwide/rustup-home" "-w" "/opt/rustwide/workdir" "-m" "6442450944" "--cpus" "6" "--user" "1001:1001" "--network" "none" "ghcr.io/rust-lang/crates-build-env/linux@sha256:cf8efcab8866f2cf4285301c7418e2f4f2a9b088c91ba69c40d5b659f81557f7" "/opt/rustwide/cargo-home/bin/cargo" "+nightly" "rustdoc" "--lib" "-Zrustdoc-map" "--config" "build.rustdocflags=[\"--cfg\", \"docsrs\", \"-Z\", \"unstable-options\", \"--emit=invocation-specific\", \"--resource-suffix\", \"-20250729-1.90.0-nightly-ba7e63b63\", \"--static-root-path\", \"/-/rustdoc.static/\", \"--cap-lints\", \"warn\", \"--extern-html-root-takes-precedence\"]" "--offline" "-Zunstable-options" "--config=doc.extern-map.registries.crates-io=\"https://docs.rs/{pkg_name}/{version}/x86_64-unknown-linux-gnu\"" "-Zrustdoc-scrape-examples" "-j6" "--target" "x86_64-unknown-linux-gnu", kill_on_drop: false }`
[INFO] [stderr] WARNING: Your kernel does not support swap limit capabilities or the cgroup is not mounted. Memory limited without swap.
[INFO] [stdout] efd0bdece441370c2fe87a772b7a26a9fa323c7e58417d89d90c332f11785411
[INFO] running `Command { std: "docker" "start" "-a" "efd0bdece441370c2fe87a772b7a26a9fa323c7e58417d89d90c332f11785411", kill_on_drop: false }`
[INFO] [stderr] warning: target filter specified, but no targets matched; this is a no-op
[INFO] [stderr] warning: [email protected]: Skipping protobuf code generation on docs.rs.
[INFO] [stderr]  Documenting spiffe v0.6.7 (/opt/rustwide/workdir)
[INFO] [stderr] error: couldn't read `/opt/rustwide/target/x86_64-unknown-linux-gnu/debug/build/spiffe-f0105de0204d5a80/out/workload.rs`: No such file or directory (os error 2)
[INFO] [stderr]    --> src/lib.rs:105:9
[INFO] [stderr]     |
[INFO] [stderr] 105 |         include!(concat!(env!("OUT_DIR"), "/workload.rs"));
[INFO] [stderr]     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[INFO] [stderr] 
[INFO] [stderr] warning: [email protected]: Skipping protobuf code generation on docs.rs.
[INFO] [stderr] error: could not document `spiffe`
[INFO] running `Command { std: "docker" "inspect" "efd0bdece441370c2fe87a772b7a26a9fa323c7e58417d89d90c332f11785411", kill_on_drop: false }`
[INFO] running `Command { std: "docker" "rm" "-f" "efd0bdece441370c2fe87a772b7a26a9fa323c7e58417d89d90c332f11785411", kill_on_drop: false }`
[INFO] [stdout] efd0bdece441370c2fe87a772b7a26a9fa323c7e58417d89d90c332f11785411