Pass through RUSTFLAGS to qemu

Fixes ignoring tests on qemu platforms (mips & powerpc)

Author: Susurrus

Cross.toml

@@ -0,0 +1,2 @@
+[build.env]
+whitelist = ["RUSTFLAGS"]

ci/install.sh

@@ -16,8 +16,7 @@ main() {
     curl -LSfs https://japaric.github.io/trust/install.sh | \
         sh -s -- \
            --force \
-           --git japaric/cross \
-           --tag v0.1.4 \
+           --git susurrus/cross \
            --target $target
 }