ci: Drop custom msrv checking

The way this tries to replace the system Rust is hacky and
actually I realized belatedly I may have broken it recently; basically
`installdeps.sh` re-adds the system one, and it's hard to be sure
with our current buildsystem we're using the newer one from `$PATH`.

What we really want to do here is use a CentOS8 buildroot,
which will automatically enforce this in a better way along
with solving other problems.  But right now we've broken
that because libdnf requires a too-new libmodulemd.

So let's just rely on the Fedora rust for now.

.cci.jenkinsfile

@@ -39,8 +39,7 @@ codestyle: {
   }
 },
 unit: {
-  // this branch runs unit tests (both Rust-based and C-based) and makes sure we compile
-  // fine against the MSRV
+  // this branch runs unit tests (both Rust-based and C-based)
   def n = 5
   cosaPod(buildroot: true, runAsUser: 0, memory: "2Gi", cpu: "${n}") {
       checkout scm

Cargo.toml

@@ -3,6 +3,12 @@ name = "rpmostree-rust"
 version = "0.1.0"
 authors = ["Colin Walters <walters@verbum.org>", "Jonathan Lebon <jonathan@jlebon.com>"]
 edition = "2018"
+# See https://rust-lang.github.io/rfcs/2495-min-rust-version.html
+# Usually, we try to keep this to no newer than current RHEL8 rust-toolset version.
+# You can find the current versions from here:
+# https://access.redhat.com/documentation/en-us/red_hat_developer_tools/1/
+# However, right now we are bumping to 1.48 so we can use https://cxx.rs
+#rust = "1.48"
 links = "rpmostreeinternals"
 
 # This currently needs to duplicate the libraries in configure.ac

ci/unit.sh

@@ -1,15 +1,7 @@
 #!/bin/bash
 set -euo pipefail
 
-# Usually, we try to keep this to no newer than current RHEL8 rust-toolset version.
-# You can find the current versions from here:
-# https://access.redhat.com/documentation/en-us/red_hat_developer_tools/1/
-# However, right now we are bumping to 1.48 so we can use https://cxx.rs
-MINIMUM_SUPPORTED_RUST_VERSION=1.48
-
 ci/installdeps.sh
-dnf remove -y cargo rust
-curl https://sh.rustup.rs -sSf | sh -s -- --default-toolchain ${MINIMUM_SUPPORTED_RUST_VERSION} -y
+ci/install-extra-builddeps.sh
 export PATH="$HOME/.cargo/bin:$PATH"
 ci/build.sh
-cargo +${MINIMUM_SUPPORTED_RUST_VERSION} test