testdata: deduplicate GODEBUG cleanup line

I added a second one recently without noticing, since the original was
too far below and I had written commands above it.

Deduplicate them, and put the new env line as early as possible, to
prevent further issues and confusion.

testdata/scripts/tiny.txt

@@ -5,6 +5,7 @@ garble -tiny build
 ! binsubstr main$exe 'garble_main.go' 'fmt/print.go'
 env GODEBUG='allocfreetrace=1,gcpacertrace=1,gctrace=1,inittrace=1,scavenge=1,scavtrace=1,scheddetail=1,schedtrace=10'
 ! exec ./main$exe
+env GODEBUG='' # Ensure further commands don't have weirdness due to GODEBUG.
 stderr '^\(0x[\d\w]{4,8},0x[\d\w]{4,8}\)' # interfaces/pointers print correctly
 # With -tiny, all line numbers are reset to 1.
 # Unfortunately, line comment directives don't allow erasing line numbers entirely.
@@ -12,13 +13,10 @@ stderr '^caller: \?\? 1$' # position info is removed
 stderr '^recovered: ya like jazz?'
 ! stderr 'panic: oh noes' # panics are hidden
 
-# Ensure further commands don't have weirdness due to GODEBUG.
-env GODEBUG=''
 
 [short] stop # no need to verify this with -short
 
 # Default mode
-env GODEBUG=
 garble build
 ! exec ./main$exe
 stderr '^caller: [0-9a-zA-Z_]+\.go [1-9]'