ci(gitlab,windows): provide GNU Rust's host-linker support

GitLab's MinGW job cannot find `x86_64-w64-mingw32-gcc` when linking
gitcore's build script:
https://gitlab.com/dscho/git1/-/jobs/16593470275

Although gitcore is a static library, Cargo first links `build.rs`
as a host executable. We omitted the Rust MSI's `Gcc` feature, which
supplies the required linker and platform libraries:
https://github.com/rust-lang/rust/blob/1.96.0/src/etc/installer/msi/rust.wxs

Assisted-by: GPT-6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ci/install-dependencies.ps1

@@ -49,7 +49,7 @@ $rustMsi = Get-Installer "rust-$rustTarget.msi" (
     "https://static.rust-lang.org/dist/" +
     "rust-$RustVersion-x86_64-pc-windows-$rustTarget.msi")
 Invoke-Installer msiexec.exe @('/i', $rustMsi, 'INSTALLDIR=C:\Rust',
-    'ADDLOCAL=Rustc,Cargo,Std', '/quiet', '/norestart')
+    "ADDLOCAL=Rustc,Cargo,Std$(if ($Mingw) { ',Gcc' })", '/quiet', '/norestart')
 
 if ($Mingw) {
     return