devtools/signup: add $SHEET_ID configuration for signup tasks
devtools/signup: add $SHEET_ID configuration for signup tasks

The $SHEET_ID environment variable is mapped to fetch.go's --sheet-id value.

MultiPart: 2/2
Change-Id: I6873eb0d3f60de54e6ba4b0cc13ed04fb098718f

vanadium/go.jiri,vanadium/go.jiri