Proofs of programs that use the `print_string` syscall are not valid

[gabrielbosio]

Tested with print.elf and commit.elf:

print.elf

% cargo run -p cli --release -- prove executor/program_artifacts/rust/print.elf -o /tmp/proof.bin
    Finished `release` profile [optimized] target(s) in 0.06s
     Running `target/release/cli prove executor/program_artifacts/rust/print.elf -o /tmp/proof.bin`
Reading ELF file...
Generating proof...
PRINT VM: Hello, World!
Writing proof...
Proof written to "/tmp/proof.bin"
% cargo run -p cli --release -- verify /tmp/proof.bin executor/program_artifacts/rust/print.elf 
    Finished `release` profile [optimized] target(s) in 0.06s
     Running `target/release/cli verify /tmp/proof.bin executor/program_artifacts/rust/print.elf`
Reading ELF file...
Reading proof...
Verifying proof...
Verification failed!

commit.elf

% echo hello > /tmp/input.bin
% cargo run -p cli --release -- prove executor/program_artifacts/rust/commit.elf --private-input /tmp/input.bin -o /tmp/proof.bin
    Finished `release` profile [optimized] target(s) in 0.17s
     Running `target/release/cli prove executor/program_artifacts/rust/commit.elf --private-input /tmp/input.bin -o /tmp/proof.bin`
Reading ELF file...
Reading private input file...
Generating proof...
PRINT VM: Private input received: [104, 101, 108, 108, 111, 10]

Writing proof...
Proof written to "/tmp/proof.bin"
% cargo run -p cli --release -- verify /tmp/proof.bin executor/program_artifacts/rust/commit.elf
    Finished `release` profile [optimized] target(s) in 0.06s
     Running `target/release/cli verify /tmp/proof.bin executor/program_artifacts/rust/commit.elf`
Reading ELF file...
Reading proof...
Verifying proof...
Verification failed!

Expected

Both programs should pass its verification, independent of the private input passed.