fix(srs): Detect shasum utility when downloading lagrange

[phated]

Description

This switches the shasum utility from sha256sum to shasum -a 256, since sha256sum is not available on a default mac system and would require coreutils to be installed. My understanding is that shasum with the 256 algorithm selected is the same thing (and is working on my nearly-default mac). shasum manpage: https://linux.die.net/man/1/shasum

Checklist:

• I have reviewed my diff in github, line by line.
• Every change is related to the PR description.
• I have linked this pull request to the issue(s) that it resolves.
• There are no unexpected formatting changes, superfluous debug logs, or commented-out code.
• There are no circuit changes, OR specifications in /markdown/specs have been updated.
• There are no circuit changes, OR a cryptographer has been assigned for review.
• I've updated any terraform that needs updating (e.g. environment variables) for deployment.
• The branch has been rebased against the head of its merge target.
• I'm happy for the PR to be merged at the reviewer's next convenience.
• New functions, classes, etc. have been documented according to the doxygen comment format. Classes and structs must have @brief describing the intended functionality.
• If existing code has been modified, such documentation has been added or updated.

[dbanks12]

Looks good to me.

[phated]

I noticed that the original download_ignition.sh actually detected which command was available, so I updated this to use the same mechanism