Document environment variable CRYSTAL_EXEC_PATH (#15631)

Author: straight-shoota

doc/man/crystal.adoc

@@ -131,6 +131,9 @@ subsequent builds. This path is also used to temporarily store executables
 when Crystal programs are run with '*crystal* run' rather than '*crystal*
 build'.
 
+=== CRYSTAL_EXEC_PATH
+Determines the path where *crystal* looks for external sub-commands.
+
 === CRYSTAL_LIBRARY_PATH
 Defines paths where Crystal searches for (binary) libraries. Multiple paths
 can be separated by ":".	 These paths are passed to the linker as `-L`

man/crystal.1

@@ -810,6 +810,9 @@ Defines path where Crystal caches partial compilation results for faster
 subsequent builds. This path is also used to temporarily store executables
 when Crystal programs are run with \*(Aq\fBcrystal\fP run\*(Aq rather than \*(Aq\fBcrystal\fP
 build\*(Aq.
+.SS "CRYSTAL_EXEC_PATH"
+.sp
+Determines the path where crystal looks for external sub-commands.
 .SS "CRYSTAL_LIBRARY_PATH"
 .sp
 Defines paths where Crystal searches for (binary) libraries. Multiple paths