Install script improvements

The current install script has bogus requirements of bash and shasum. Because of the way it is written the script runs fine with sh and sha1sum.

This should be fixed because some environments, for example the Alpine Linux docker image, do not have them.
If you are checking for curl and wget, you should also check for sha1sum (or better yet use sha256sum).

The hard requirement on bash is just unneeded.

The requirements can easily be bypassed by just faking the environment, proving my point:

```sh
cat <(echo "BASH_VERSION=fake; function shasum() { sha1sum $@; }") <(wget -O- https://carvel.dev/install.sh) | sh
```

Read the install.sh entry point and inspect how it checks for bash, shasum, curl, and wget. Run it with sh in an Alpine Linux Docker image, then confirm the requirements match the commands the script actually uses and that the documented environment checks pass.

Written by the indexing model from the issue text.