build.sh: Fixed incorrect tag detection logic in repo init.

Description

build.sh: Fixed incorrect tag detection logic in repo init.

• Previously, repo init used substring matching for tags, leading to false positives (e.g., "a14" matched "v2.8-beta.2-a14").
• Now, we fetch all tags and check for an exact match using grep -Fx.
• Added repo init command logging for better view on repo init command
• Ensure a clean reset of the default manifest before repo initialization

Issue(s)

https://gitlab.e.foundation/e/os/backlog/-/issues/2989

Test procedures

• Trigger a build on a14 branch with different BRANCH_NAME values (e.g. a14, v2.8-beta.2-a14).
• Verify that repo init logs the correct command and initializes with the right branch/tag.

Test Builds

With Branch name: https://gitlab.e.foundation/e/priv/os/build/-/jobs/1129975

With Tag name: https://gitlab.e.foundation/e/priv/os/build/-/jobs/1129958 (Ignore failure as that is due to derp on OS source side.) Observe repo init command, it correctly fetches tag.

Check list

• Self review
• Test procedure explained
• Tested on fresh install
• Tested applied as an update
• License
• Internal documentation
• User documentation