Fix get script for tags

[saschagrunert]

What type of PR is this?

/kind bug

What this PR does / why we need it:

We have to specify the right git ref for cosign to let the script work correctly on tags.

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

None

Does this PR introduce a user-facing change?

Fixed correct git ref usage for get script when being used on tags.

[openshift-ci]

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This pull-request has been approved by: saschagrunert

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[saschagrunert]

@rphillips @haircommander PTAL, works now as intended:

> ./scripts/get -t 4cd8c485758ad7bb5413e8b35903cff18bdc4b49
Welcome to the conmon-rs install script!
Using commit: 4cd8c485758ad7bb5413e8b35903cff18bdc4b49
Checking if all commands are available: curl jq
Found commit: 4cd8c485758ad7bb5413e8b35903cff18bdc4b49
Using git ref: refs/heads/main
Found cosign, verifying binary signature
Verified OK
Installed binary into: conmonrs

version: 0.5.1
tag: none
commit: 4cd8c485758ad7bb5413e8b35903cff18bdc4b49
build: 2023-08-31 08:07:13 +00:00
target: x86_64-unknown-linux-gnu
rustc 1.72.0 (5680fa18f 2023-08-23)
cargo 1.72.0 (103a7ff2e 2023-08-15)

> ./scripts/get -l v0.6.0
Welcome to the conmon-rs install script!
Using tag: v0.6.0
Checking if all commands are available: curl jq
Getting commit from tag
Found commit: 38d8e61c5210c8c917e35704fb862fda05d6fe5e
Using git ref: refs/tags/v0.6.0
Found cosign, verifying binary signature
Verified OK
Installed binary into: conmonrs

version: 0.6.0
tag: v0.6.0
commit: 38d8e61c5210c8c917e35704fb862fda05d6fe5e
build: 2023-09-04 06:58:55 +00:00
target: x86_64-unknown-linux-gnu
rustc 1.72.0 (5680fa18f 2023-08-23)
cargo 1.72.0 (103a7ff2e 2023-08-15)

> ./scripts/get
Welcome to the conmon-rs install script!
Checking if all commands are available: curl jq
Getting latest commit on main
Commit 38d8e61c5210c8c917e35704fb862fda05d6fe5e is tag ref
Found commit: 38d8e61c5210c8c917e35704fb862fda05d6fe5e
Using git ref: refs/tags/v0.6.0
Found cosign, verifying binary signature
Verified OK
Installed binary into: conmonrs

version: 0.6.0
tag: v0.6.0
commit: 38d8e61c5210c8c917e35704fb862fda05d6fe5e
build: 2023-09-04 06:58:55 +00:00
target: x86_64-unknown-linux-gnu
rustc 1.72.0 (5680fa18f 2023-08-23)
cargo 1.72.0 (103a7ff2e 2023-08-15)

[saschagrunert]

@haircommander PTAL

[haircommander]

/lgtm