mtr: environment MTR_MEM conflicts with --mem

Since 716621db3f3055781e24f561325cec6eac181717, opt_mem is
a pure flag. However it is assigned to the value of $MTR_MEM.

This leads to conflicts when MTR_MEM=/xxx/yy ./mtr --mem is
invoked. Here the --mem option overrided opt_mem leaving the
default path to be chosen rather than $MTR_MEM.