new non-determinism in linker maps introduced recently

PR 141769 looks like it introduced non-determinism in the generation of dylibs -- it is looking like the tmpdir is being introduced into the linker map file -- perhaps when creating the .rustc section here: https://github.com/rust-lang/rust/blob/3129d37ef7075ee3cbaa3d6cbe1b5794f67192b0/compiler/rustc_codegen_ssa/src/back/metadata.rs#L571

We are seeing it in determinism-validation builds in Fuchsia.

Note: the original description claimed that the .so files had also changed, hence some of the discussion below.
Although the .so files are identical, we are seeing .so.map files differing, with distinct tmp paths, e.g.:

            f91ef            f91ef      431     1         .../rust/linux-x64/lib/rustlib/x86_64-unknown-fuchsia/lib/libcompiler_builtins-84c11efc675a4b65.rlib(compiler_builtins-84c11efc675a4b65.compiler_builtins.ac0a882909fb5ff0-cgu.018.rcgu.o):(.debug_line)
            f9620            f9620      30d     1         obj/zircon/system/ulib/sync/libsync.a(libsync.mutex.c.o):(.debug_line)
                0                0   1cf28a     1 .rustc
-               0                0   1cf28a     1         /tmp/rustcli35e4/rmeta.o:(.rustc)
+               0                0   1cf28a     1         /tmp/rustcvBrdn3/rmeta.o:(.rustc)
                0                0   1cf28a     1                 rust_metadata_vfs_3772e3b421ca7a16
                0                0      187     1 .debug_loclists
                0                0      22b     1         obj/zircon/system/ulib/sync/libsync.a

Suggested label: A-reproducibility

Start with compiler/rustc_codegen_ssa/src/back/metadata.rs around line 571 and compare the changes from PR 141769. Reproduce the determinism-validation linker-map difference and trace how the .rustc section receives the temporary path. Done means identical .so.map outputs across builds without tmpdir-dependent entries.