Install script should use something other than `shasum`

**Describe the problem/challenge you have**
I was trying to create a docker container to use ytt during a ci/cd operation, and found that the install script wants 'shasum` to be installed, which is apparently part of perl-utils in alpine, which of course also requires perl.

```
/ # apk add shasum
fetch https://dl-cdn.alpinelinux.org/alpine/v3.14/main/x86_64/APKINDEX.tar.gz
fetch https://dl-cdn.alpinelinux.org/alpine/v3.14/community/x86_64/APKINDEX.tar.gz
ERROR: unable to select packages:
shasum (no such package):
required by: world[shasum]
/ # apk add perl-utils
(1/3) Installing libbz2 (1.0.8-r1)
(2/3) Installing perl (5.32.1-r0)
(3/3) Installing perl-utils (5.32.1-r0)
Executing busybox-1.33.1-r3.trigger
OK: 43 MiB in 17 packages
```

As it shows above, this results in 43MiB larger docker image, unnecessarily.

**Describe the solution you'd like**

I love that the install script is checking SHA sums, but maybe we could use something other than "shasum" executable?

Maybe `sha256sum` directly?

**Anything else you would like to add:**

I know there are other workarounds... just a suggestion/recommendation.

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Locate the install script and find where it invokes `shasum`; first compare the available checksum tools in the Alpine container described in the issue. Done means the script still verifies downloads while avoiding the unnecessary Perl-based dependency, with the install path or relevant checks rerun to confirm it works.

Written by the indexing model from the issue text.