A $ in a make variable must be doubled

Trigger: A job passes a -run pattern through make testacc "TESTARGS=-run '<pattern>'", and the pattern contains $. The log shows the command without the end of the pattern and /bin/sh: 1: Syntax error: Unterminated quoted string.

Fix: Pass the pattern in an env var, and double every $ before it reaches make:

env:
          RUN_PATTERN: ${{ matrix.pattern }}
        run: make testacc "TESTARGS=-run '${RUN_PATTERN//\$/\$\$}'"

Check the result with make -n testacc "TESTARGS=...". A pattern checked with go test directly does not prove that it works through make.

Why: make expands $x in a variable value. In '^TestX$', it reads $' as an empty variable, so the closing quote disappears. $$ reaches the shell as one $.