Building projects on the command line using the Bare-Arduino-Project template
You  probably look at the wrong boards.txt:

# Trinket 5V Configuration
trinket5.name=Adafruit Trinket (ATtiny85 @ 16MHz)


this can be found in the boards.txt in the main directory

C:\Users\YOUR_USER_NAME\AppData\Local\Arduino15\packages\arduino\hardware


in the sub directory 

\adafruit\avr 

in the same main directory should  also be a folder named 1.8.2 (for ArduinioIDE 1.8.12) for the core files

if added via board-manager using this line in preferences/additional bord manager urls

https://adafruit.github.io/arduino-board-index/package_adafruit_index.json


If you did a manual installation check with this guide for pit traps.Details on using trinket with the avrdude command line can be found here