tests: ensure pack binary is installed correctly when running tests in a devcontainer

.devcontainer/Dockerfile

@@ -2,7 +2,7 @@ FROM dokku/dokku:latest
 
 RUN apt-get update
 RUN apt-get install --no-install-recommends -y build-essential file nano && \
-  apt-get install --no-install-recommends -y help2man shellcheck uuid-runtime wget xmlstarlet && \
+  apt-get install --no-install-recommends -y software-properties-common help2man shellcheck uuid-runtime wget xmlstarlet && \
   apt-get clean autoclean && \
   apt-get autoremove --yes && \
   rm -rf /var/lib/apt/lists/*

tests/unit/test_helper.bash

@@ -637,7 +637,7 @@ convert_to_dockerfile() {
 }
 
 install_pack() {
-  if ! command -v "pack" &>/dev/null; then
+  if [[ ! -x /usr/bin/pack ]]; then
     add-apt-repository --yes ppa:cncf-buildpacks/pack-cli
     apt-get update
     apt-get --yes install pack-cli