Build submissions with toolchain from Alire (#354)

* New workflow to test crates using indexed toolchain

* Conditionally install compiler

* Print crate environment after build

scripts/gh-build-crate.sh

@@ -144,8 +144,10 @@ for file in $CHANGES; do
    elif $is_binary; then
       echo FETCHED BINARY crate OK
    else
-      echo LISTING EXECUTABLES of crate $milestone
       cd ${crate}_${version}_*
+      echo BUILD ENVIRONMENT
+      alr printenv
+      echo LISTING EXECUTABLES of crate $milestone
       alr run -d --list
       cd ..
    fi