Sync flags in docs.yml with package.metadata.docs.rs (#14734)

# Objective

This PR is a follow-up to #14703. I forgot to also add the flags from
`package.metadata.docs.rs` to the docs build.

## Solution

As [discussed on
Discord](https://discord.com/channels/691052431525675048/1272629407659589663/1272643734260940940),
I added the missing flags to `docs.yml`.

I also updated `rustc-args` to properly make use of the array (I think
the value has to be either a space-separated string *or* an array of
strings but not an array of space-separated strings 😆)

.github/workflows/docs.yml

@@ -58,6 +58,7 @@ jobs:
       - name: Build docs
         env:
           # needs to be in sync with [package.metadata.docs.rs]
+          RUSTFLAGS: --cfg docsrs_dep
           RUSTDOCFLAGS: -Zunstable-options --cfg=docsrs --generate-link-to-definition
         run: |
           cargo doc \

Cargo.toml

@@ -3368,7 +3368,7 @@ panic = "abort"
 # impls for re-exported traits. See https://github.com/rust-lang/cargo/issues/8811
 # for details on why this is needed. Since dependencies don't expect to be built
 # with `--cfg docsrs` (and thus fail to compile) we use a different cfg.
-rustc-args = ["--cfg docsrs_dep"]
+rustc-args = ["--cfg", "docsrs_dep"]
 rustdoc-args = ["-Zunstable-options", "--generate-link-to-definition"]
 all-features = true
 cargo-args = ["-Zunstable-options", "-Zrustdoc-scrape-examples"]