Rollup merge of #142026 - smpdt:master, r=Kobzol
bootstrap: Fix file permissions when dereferencing symlinks
## Problem
When copying files in the bootstrap process with `dereference_symlinks = true`, we're incorrectly using the symlink's metadata to set permissions on the copied regular file, which results in the following error:
```
Warning: Failed to set file times for "/build/nix-build-rustc-1.86.0.drv-0/rustc-1.86.0-src/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/bin/llvm-strip" (permissions: Permissions(FilePermissions { mode: 0o100000 (----------) })) error: Permission denied (os error 13)
```
Verbose Logs confirming the error:
```
TRACE: Found llvm-strip copy operation
Source: /n/nix/tech/store/n34yzv2n50p6lbjmx089vjym121wbl4j-llvm-19.1.7/bin/llvm-strip
Destination: /build/nix-build-rustc-1.86.0.drv-0/rustc-1.86.0-src/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/bin/llvm-strip
Source is_symlink (via symlink_metadata): true
Source symlink_metadata permissions: 120000
Source symlink_metadata file_type: FileType { is_file: false, is_dir: false, is_symlink: true, .. }
Source is symlink pointing to: llvm-objcopy
Source raw mode: 120000
Source filetype: 120000
Setting permissions: Permissions(FilePermissions { mode: 0o120000 (l---------) })
Permission mode to set: 120000
Raw permission bits: 120000
File type bits being set: 120000
Permission bits being set: 0
WARNING: Attempting to set symlink file type (120000) on regular file!
WARNING: Setting zero permission bits will make file inaccessible!
Destination permissions after set_permissions: 100000
```
## Solution
After canonicalizing a symlink path, fetch the metadata of the target file instead of continuing to use the symlink's metadata. This ensures:
- Correct file type detection
- Proper permission bits for the target file
- Maintains existing behavior for non-symlink cases
## Testing
Verified fix resolves permission errors:
```
rustc> llvm-strip: Original metadata mode: 120000, is_symlink: true
rustc> llvm-strip: Target metadata mode after fix: 100555
rustc> llvm-strip: Final permissions mode: 100555
```
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