Fix and clean up get_tag tag lookup (#16)

* fix: quote grep patterns in get_tag to match version delimiter correctly

The unquoted \. was likely a typo for a literal dot, but it expands to .
(any char), which silently matches the - delimiter after the version.
Quote the patterns and use an explicit - to match the actual tag format.

* refactor: use braces instead of subshell in get_tag

No need to fork a subshell for the grep fallback; a brace group works
the same way and avoids the overhead.

Author: arkamar

build.sh

@@ -20,11 +20,13 @@ function install_dependencies {
 
 # prints the git tag corresponding to the newest and best matching the provided kernel version $1
 function get_tag {
-    local KERNEL_VERSION=$1
+    local KERNEL_VERSION="${1}"
 
     # list all tags from newest to oldest
-    (git --no-pager tag -l --sort=-creatordate | grep microvm-kernel-$KERNEL_VERSION\..*\.amzn2 \
-        || git --no-pager tag -l --sort=-creatordate | grep kernel-$KERNEL_VERSION\..*\.amzn2) | head -n1
+    {
+        git --no-pager tag -l --sort=-creatordate | grep "microvm-kernel-${KERNEL_VERSION}-.*\.amzn2" \
+        || git --no-pager tag -l --sort=-creatordate | grep "kernel-${KERNEL_VERSION}-.*\.amzn2"
+    } | head -n1
 }
 
 function build_version {