ci: switch to nightly rustc

Was: Added FFIVector to get Polonius output on C++ side
This was a commit with changes to both to-upstream and not-to-upstream files.
This is the "not-to-upstream" part. The other part is already installed upstream.

ChangeLog:

	* .github/workflows/ccpp.yml: Switch to nightly version of rustc
	to use unstable features, namely extern types for FFI.

Signed-off-by: Kushal Pal <[email protected]>

Author: braw-lee

.github/workflows/ccpp.yml

@@ -42,7 +42,7 @@ jobs:
                   g++-multilib \
                   dejagnu;
           # install Rust directly using rustup
-          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain=1.72.0;
+          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain=nightly;
 
     - name: Configure
       run: |
@@ -123,7 +123,7 @@ jobs:
                   g++-multilib \
                   dejagnu;
           # install Rust directly using rustup
-          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain=1.72.0;
+          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain=nightly;
 
     - name: Configure
       run: |
@@ -206,7 +206,7 @@ jobs:
                   g++-4.8-multilib \
                   dejagnu;
           # install Rust directly using rustup
-          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain=1.72.0;
+          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain=nightly;
 
     - name: Configure
       run: |
@@ -265,7 +265,7 @@ jobs:
       run: |
           brew install dejagnu mpfr libmpc gmp;
           # install Rust directly using rustup
-          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain=1.72.0;
+          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain=nightly;
 
     - name: Configure
       run: |