Fix option --is-view env var

[leighmcculloch]

What

Change the cli --is-view env var to be SOROBAN_INVOKE_VIEW instead of SYSTEM_TEST_VERBOSE_OUTPUT, retaining support for the previous env var for backwards compatibility.

Why

In #1194 when the env var was added it appears we accidentally named it a name that doesn't have anything to do with the option. The env var is actually the name of an env var used in the system test suite, so I'm going to guess that a find all and rename went awry and maybe renamed an env var here that shouldn't have been.

At the moment two env vars are attached to the option, and only the second one appears to be useable in my testing, as the second env parameter overrides the first.

This change is not a breaking change and can be released in a patch release as a bug fix.

The old env var is not advertised in the help output.

Example

Before

      --is-view
          Do not sign and submit transaction
          
          [env: SYSTEM_TEST_VERBOSE_OUTPUT=]

After

      --is-view
          View the result simulating and do not sign and submit transaction
          
          [env: SOROBAN_INVOKE_VIEW=]