ci: fix musl docker build

Looks like the old japaric images are bunk. We update our docker image
to be based on the new rustembedded images and configure cross to use
it.

Turns out that this wasn't due to a stale docker image, but rather, a
bug in cross: cross-rs/cross#357
We work around that bug by installing the master branch of cross. Sigh.

Cross.toml

@@ -1,2 +1,2 @@
 [target.x86_64-unknown-linux-musl]
-image = "burntsushi/x86_64-unknown-linux-musl:v0.1.14"
+image = "burntsushi/cross:x86_64-unknown-linux-musl"

ci/docker/x86_64-unknown-linux-musl/Dockerfile

@@ -1,4 +1,4 @@
-FROM japaric/x86_64-unknown-linux-musl:v0.1.14
+FROM rustembedded/cross:x86_64-unknown-linux-musl
 
 RUN apt-get update \
  && apt-get install -y --no-install-recommends \

ci/utils.sh

@@ -99,7 +99,9 @@ is_osx() {
 
 builder() {
     if is_musl && is_x86_64; then
-        cargo install cross
+        # cargo install cross
+        # To work around https://github.com/rust-embedded/cross/issues/357
+        cargo install --git https://github.com/rust-embedded/cross --force
         echo "cross"
     else
         echo "cargo"