firefox ftbfs with cosmic toolchain updates

[Matthias Klose]

https://launchpadlibrarian.net/418304928/buildlog_ubuntu-cosmic-amd64.firefox_63.0+build1-0ubuntu1_BUILDING.txt.gz

using ppa:ubuntu-toolchain-r/ppa

   Compiling toml v0.4.8
   Compiling cbindgen v0.6.4 (/<<BUILDDIR>>/firefox-63.0+build1/third_party/cbindgen)
    Finished release [optimized] target(s) in 2m 42s
warning: Using `cargo install` to install the binaries for the package in current working directory is deprecated, use `cargo install --path .` instead. Use `cargo build` if you want to simply build the package.
  Installing cbindgen v0.6.4 (/<<BUILDDIR>>/firefox-63.0+build1/third_party/cbindgen)
error: failed to compile `cbindgen v0.6.4 (/<<BUILDDIR>>/firefox-63.0+build1/third_party/cbindgen)`, intermediate artifacts can be found at `/<<BUILDDIR>>/firefox-63.0+build1/third_party/cbindgen/target`

Caused by:
  failed to open: /sbuild-nonexistent/.cargo/registry/index/github.com-1ecc6299db9ec823/.cargo-index-lock

Caused by:
  Permission denied (os error 13)
make: *** [/<<BUILDDIR>>/firefox-63.0+build1/debian/build/rules.mk:271: cbindgen/bin/cbindgen] Error 101
dpkg-buildpackage: error: debian/rules build subprocess returned exit status 2

[Matthias Klose (doko)]

not seen in the updates pocket