Fix and clean up get_tag tag lookup

[arkamar]

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.

Also replace the unnecessary subshell with a brace group.

[cursor]

PR Summary

Low Risk
Low risk: only adjusts tag-matching logic in the build script, affecting which git tag is checked out for a given kernel version.

Overview
Fixes build.sh get_tag tag selection by quoting the grep patterns and matching an explicit - delimiter after the kernel version (instead of the previous loosely-escaped \.), ensuring the newest tag chosen corresponds to the intended microvm-kernel-${version}-...\.amzn2 or kernel-${version}-...\.amzn2 format.

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