feat(c-api) Do not build C headers if the env var DOCS_RS exists

[Hywan]

Description

Fix #1954.

According to https://docs.rs/about/builds, the DOCS_RS environment variable is set when the documentation is built from docs.rs. In this case, we must not build the C headers because it generates such error (see #1954 to learn more):

Unable to copy the generated C bindings: Os { code: 30, kind: Other, message: "Read-only file system" }

Review

• Add a short description of the the change to the CHANGELOG.md file

[MarkMcCaskey]

This is an improvement but it's still broken when doing cargo publish; we'll have to either cargo publish --no-verify or DOCS_RS=1 cargo publish; currently it's the 1 crate not covered by publish.py: we have to publish it manually.

(An easy fix would be to extend publish.py to allow setting env vars)

[Hywan]

Let's merge this PR for the moment, and let's add another to solve the cargo publish problem :-).

bors r+

[bors]

Build succeeded:

• Audit
• Code lint
• Test cross-compile on linux
• Test cross-compile on macos
• Test on linux-aarch64
• Test on linux-x64
• Test on macos-x64
• Test on windows-x64