make-bootstrap-tools.nix: don't pull in pkgs.glibc in test (#210038)

`builtins.baseNameOf` retains any string context, causing the test
derivation to incorrectly depend on `pkgs.glibc`. All we really want is
to know what the dynamicLinker is called, but we don't need it to be
present in store.

Thanks to Adam Joseph for spotting this.

pkgs/stdenv/linux/make-bootstrap-tools.nix

···
       gcc --version
 
     '' + lib.optionalString (stdenv.hostPlatform.libc == "glibc") ''
-      ldlinux=$(echo ${bootstrapTools}/lib/${builtins.baseNameOf binutils.dynamicLinker})
+      ldlinux=$(echo ${bootstrapTools}/lib/${builtins.unsafeDiscardStringContext /* only basename */ (builtins.baseNameOf binutils.dynamicLinker)})
       export CPP="cpp -idirafter ${bootstrapTools}/include-glibc -B${bootstrapTools}"
       export CC="gcc -idirafter ${bootstrapTools}/include-glibc -B${bootstrapTools} -Wl,-dynamic-linker,$ldlinux -Wl,-rpath,${bootstrapTools}/lib"
       export CXX="g++ -idirafter ${bootstrapTools}/include-glibc -B${bootstrapTools} -Wl,-dynamic-linker,$ldlinux -Wl,-rpath,${bootstrapTools}/lib"