CMake: Fix BLS signature library dependence on $HOME

Instead, allow the build system to provide Cargo, and rely on the build
system to set the PATH appropriately.

We could possibly keep the error message if cargo is not found, but I
believe the error output of add_custom_target() will be useful enough if
cargo is missing.

Author: eigendude

deps/bls-signatures.cmake

@@ -3,17 +3,6 @@
 # SPDX-License-Identifier: Apache-2.0
 #
 
-if(EXISTS "$ENV{HOME}/.cargo/bin/cargo")
-  message(STATUS "Rust cargo is present")
-else()
-  message(STATUS "No Rust cargo found")
-  message(WARNING
-          "Rust is required for building BLS dependencies.\n"
-          "You can install rust-lang via the following command:\n"
-          "curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh"
-  )
-endif()
-
 set(BUILD_TYPE "release")
 
 set(BLS_BUILD_PATH
@@ -27,7 +16,7 @@ set(BLS_LIBRARY
 add_custom_target(
     cargo_build
     ALL
-    COMMAND $ENV{HOME}/.cargo/bin/cargo build -p bls-signatures-ffi --target-dir ${BLS_BUILD_PATH} --${BUILD_TYPE}
+    COMMAND cargo build -p bls-signatures-ffi --target-dir ${BLS_BUILD_PATH} --${BUILD_TYPE}
     WORKING_DIRECTORY "${PROJECT_SOURCE_DIR}/deps/bls-signatures"
     BYPRODUCTS ${BLS_LIBRARY}
   )