ci/coverage: Fix cargo-tarpaulin version to 0.26.1

The nightly coverage build is currently failing as it is failing to
compile the cargo-tarpaulin crate 0.27.0 with the rust compiler
1.66 (our current MSRV). We cannot upgrade the current MSRV because
certain distros need that version, so we can only:

 * Fix the version of cargo-tarpaulin to 0.26.1

This version should be unfixed when the MSRV upgrade is performed.

Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>

ci.sh

@@ -253,7 +253,8 @@ if [ "$PROVIDER_NAME" = "coverage" ]; then
     EXCLUDES="fuzz/*,e2e_tests/*,src/providers/cryptoauthlib/*,src/authenticators/jwt_svid_authenticator/*"
     UNIT_TEST_FEATURES="unix-peer-credentials-authenticator,direct-authenticator"
     # Install tarpaulin
-    cargo +${MSRV} install cargo-tarpaulin
+    # TODO: Stop using the --version parameter when MSRV is upgraded.
+    cargo +${MSRV} install cargo-tarpaulin --version 0.26.1
 
     mkdir -p reports