environment variables -> environment

As @sanxiyn says,
rust-lang#23527 (comment)

Author: steveklabnik

man/rustc.1

@@ -242,7 +242,7 @@ full debug info with variable and type information.
 \fBopt\-level\fR=\fIVAL\fR
 Optimize with possible levels 0\[en]3
 
-.SH ENVIRONMENT VARIABLES
+.SH ENVIRONMENT
 
 Some of these affect the output of the compiler, while others affect programs
 which link to the standard library.