The "test-var-set" command:

Usage: fossil test-var-set VAR ?VALUE? ?--file FILE?

Store VALUE or the content of FILE (exactly one of which must be
supplied) into variable VAR.  Use a FILE of "-" to read from
standard input.

WARNING: changing the value of a variable can interfere with the
operation of Fossil.  Be sure you know what you are doing.

Use "--blob FILE" instead of "--file FILE" to load a binary blob
such as a GIF.