Avoid "Could not find compiler set in environment variable CC" Error

[sense112]

Suggestion to update the cmake command to avoid C compiler errors

While trying to setup my firedancer node I ran into a strange issue. While trying to run the ./deps.sh command I was met with the following error message:

[+] Configuring secp256k1
CMake Error at /usr/share/cmake-3.22/Modules/CMakeDetermineCCompiler.cmake:49 (message):
  Could not find compiler set in environment variable CC:

  /usr/bin/gcc.
Call Stack (most recent call first):
  CMakeLists.txt:10 (project)


CMake Error: CMAKE_C_COMPILER not set, after EnableLanguage
-- Configuring incomplete, errors occurred!
See also "/home/ubuntu/repos/firedancer/opt/git/secp256k1/build/CMakeFiles/CMakeOutput.log".

While I understand this is likely due to a misconfiguration from my side installing the prior dependencies, the change in this PR to explicitly set the C_COMPILER during the cmake command solved my issue.

This may come in helpful to future validators.