tkey-runapp: update tkeyclient to detect Castor

[dehanj]

Description

Update so tkey-runapp can detect and load apps to a TKey running Castor.

Fixes #14 #5

Type of change

Please tick any that are relevant to this PR and remove any that aren't.

• Feature (non breaking change which adds functionality)

Submission checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my changes
• I have tested and verified my changes on target
• My changes are well written and CI is passing
• I have squashed my work to relevant commits and rebased on main for linear history
• I have added a "Co-authored-by: x" if several people contributed, either pair programming or by squashing commits from different authors.
• I have updated the documentation where relevant (readme, dev.tillitis.se etc.)
• QEMU is updated to reflect changes

[mchack-work]

• Remove toolchain from go.mod
• Ran go mod tidy

Squashed into earlier commit.