include missing env var in error message

Dependencies

• [2] 2WEO7OZL Version updates: getting rid of anyhow + moving to Tokio 1.0
• [3] ATOFE4ZX more detailed error message when SSH_AUTH_SOCK points to a bad location
• [4] 7FRJYUI6 Reboot because of a bad change

Change contents

• replacement in thrussh-keys/src/lib.rs at line 125
  [2.360]→[2.360:407](∅→∅)
  − #[error("Environment variable not found")]
  [2.360]

  [2.407]
  + #[error("Environment variable `{0}` not found")]