Redmine#6189: Store the hardcoded bootstrap file in a variable instead o...

...f using it directly to prevent old GCCs from failing due to fprintf usage as a macro

Ticket: https://dev.cfengine.com/issues/6189
Explaining what I take it @kacfengine amended in merging:
Although `char * var = "literal";` is a common idiom in C, it is actually type-unsound: a string literal should always be treated as const-qualified, since compilers and linkers may chose to put all instances of a given literal into the same piece of (potentially read-only) memory.  So a variable initialised to a string literal should be `const char * var = "literal";` (the same is in fact true of returns from getenv(), despite the function's return being `char *` for historical reasons).
Note that an array initializer, `char arr[] = "initializer";` looks similar but is just data with which to fill the array (incidentally determining its size, when unspecified, as here), so not subject to the same constraint; the array need not be const.