.cargo/config.toml: exclude from tarball

Various commits have added bits to .cargo/config.toml. Unfortunately,
this file needs to be changed by the Linux package builds (debuild, RPM,
OBS etc) with the results of `cargo vendor`, to support building in
isolated environments.

These environments - especially Debian's dpkg-buildpackage/debuild - do
not make it easy to alter a file which already exists in the tarball in
an automatic way. dpkg-buildpackage in particular requires all changes
to be made in the form of patches.

Just exclude .cargo/config.toml from the tarballs for now. This means
that the stanzas it includes _will not apply_ to builds made from
tarballs, which includes releases and development builds made using the
OBS/Launchpad PPAs.

.cargo/config.toml

@@ -1,3 +1,8 @@
+# This file is _not_ included in the tarballs for now
+# Binary builds on Linux packaging infrastructure need to overwrite it to make `cargo vendor` work
+# Releases and development builds made using OBS/Launchpad will _not_ reflect the contents of this
+# file
+
 [resolver]
 # Make cargo 1.84+ respect MSRV (rust-version in Cargo.toml)
 incompatible-rust-versions = "fallback"

.gitattributes

@@ -21,6 +21,8 @@
 /.github/* export-ignore
 /.builds export-ignore
 /.builds/* export-ignore
+# to make cargo vendor work correctly
+/.cargo/config.toml
 
 # for linguist; let github identify our project as C++ instead of C due to pcre2
 pcre2/** linguist-vendored