ci: replace cargo rustc -> cargo build to support virtual manifests

This is the last commit of the cherry-pick from [1].

`cargo rustc` is incompatible with virtual manifests, so the CI needs to
 use cargo build instead. However, passing `RUSTFLAGS="-D warnings"` via
 the environment would propagate to all dependencies, and some of them
 currently fail to build under ``-D warnings` due to issues like [0]:

```
error: creating a mutable reference to mutable static
  --> src/temp.rs:97:5
   |
97 |     DIRS.pop()
   |     ^^^^^^^^^^ mutable reference to mutable static
```

To resolve this, apply ``-D warnings` only to the `cargo clippy`
commands (which apply to our workspace only) and avoid enforcing it for
the entire cargo build.

[0]: https://github.com/cloud-hypervisor/cloud-hypervisor/actions/runs/19962283528/job/57245376263?pr=7525
[1]: cloud-hypervisor#7525

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>
On-behalf-of: SAP philipp.schuster@sap.com

Author: phip1611

.github/workflows/build.yaml

@@ -30,40 +30,40 @@ jobs:
           target: ${{ matrix.target }}
 
       - name: Build (default features)
-        run: cargo rustc --locked --bin cloud-hypervisor -- -D warnings
+        run: cargo build --locked --bin cloud-hypervisor
 
       - name: Build (kvm)
-        run: cargo rustc --locked --bin cloud-hypervisor --no-default-features --features "kvm"  -- -D warnings
+        run: cargo build --locked --bin cloud-hypervisor --no-default-features --features "kvm"
 
       - name: Build (default features + tdx)
-        run: cargo rustc --locked --bin cloud-hypervisor --features "tdx" -- -D warnings
+        run: cargo build --locked --bin cloud-hypervisor --features "tdx"
 
       - name: Build (default features + dbus_api)
-        run: cargo rustc --locked --bin cloud-hypervisor --features "dbus_api" -- -D warnings
+        run: cargo build --locked --bin cloud-hypervisor --features "dbus_api"
 
       - name: Build (default features + guest_debug)
-        run: cargo rustc --locked --bin cloud-hypervisor --features "guest_debug" -- -D warnings
+        run: cargo build --locked --bin cloud-hypervisor --features "guest_debug"
 
       - name: Build (default features + pvmemcontrol)
-        run: cargo rustc --locked --bin cloud-hypervisor --features "pvmemcontrol" -- -D warnings
+        run: cargo build --locked --bin cloud-hypervisor --features "pvmemcontrol"
 
       - name: Build (default features + fw_cfg)
-        run: cargo rustc --locked --bin cloud-hypervisor --features "fw_cfg" -- -D warnings
+        run: cargo build --locked --bin cloud-hypervisor --features "fw_cfg"
 
       - name: Build (default features + ivshmem)
-        run: cargo rustc --locked --bin cloud-hypervisor --features "ivshmem" -- -D warnings
+        run: cargo build --locked --bin cloud-hypervisor --features "ivshmem"
 
       - name: Build (mshv)
-        run: cargo rustc --locked --bin cloud-hypervisor --no-default-features --features "mshv"  -- -D warnings
+        run: cargo build --locked --bin cloud-hypervisor --no-default-features --features "mshv"
 
       - name: Build (sev_snp)
-        run: cargo rustc --locked --bin cloud-hypervisor --no-default-features --features "sev_snp"  -- -D warnings
+        run: cargo build --locked --bin cloud-hypervisor --no-default-features --features "sev_snp"
 
       - name: Build (igvm)
-        run: cargo rustc --locked --bin cloud-hypervisor --no-default-features --features "igvm"  -- -D warnings
+        run: cargo build --locked --bin cloud-hypervisor --no-default-features --features "igvm"
 
       - name: Build (mshv + kvm)
-        run: cargo rustc --locked --bin cloud-hypervisor --no-default-features --features "mshv,kvm"  -- -D warnings
+        run: cargo build --locked --bin cloud-hypervisor --no-default-features --features "mshv,kvm"
 
       - name: Release Build (default features)
         run: cargo build --locked --all --release --target=${{ matrix.target }}