rust-debian.sh: fix set -e interaction with nightly rust detection (#1592)

Currently feature(rust) in devcontainer definition fails
container build unless user specifies nightly rust.

Issue was introduced in commit 10bf6f7
#1554

Lack of "nightly" in RUST_VERSION will result in nonzero return code
from grep. This in turn triggers -e option set for the whole script.
As a result build silently fails.

Author: tomaszjonak

script-library/rust-debian.sh

@@ -183,8 +183,8 @@ else
             apt_get_update_if_needed
             apt-get -y install --no-install-recommends git
         fi
-        echo ${RUST_VERSION} | grep -q "nightly"
-        is_nightly=$?
+        is_nightly=0
+        echo ${RUST_VERSION} | grep -q "nightly" || is_nightly=$?
         if [ $is_nightly = 0 ]; then
             check_nightly_version_formatting RUST_VERSION
         else